20.5 M - Thierry Karsenti

Transcription

COMPULSORY
1
READINGS
1
According to the author of the module, the compulsory readings do not infringe known copyright.
11. COMPULSORY READINGS
An illustration of a fruit fly Drosophila melanogaster (SEM X60) taken from
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookgeninteract.htm where
permission was given by Dennis Kunkel at www.DennisKunkel.com to use the image
in the former website.
The following copyright regulations to Cell Biology by Dalton, et al. apply: You may
copy and distribute the document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license
notice saying this License applies to the Document are reproduced in all copies,
and that you add no other conditions whatsoever to those of this License.
Reading 1:
Complete reference: The structure and function of Prokaryotic and Eukaryotic
cells.
1. The Study Guide to the Science of Botany that is used in this section of the
work is a textbook at Wikibooks shelved under Biology and intended to establish
a course of study in the subject of Botany, utilizing articles provided in
Wikipedia(http://www.Wikipedia.org/), with links to other relevant web sites and
other Wikibooks as appropriate. In some cases, portions of the text from
Wikipedia articles have been used to materially develop introductory text within
2. Cell Biology, Mark Dalton and others http://en.wikibooks.org/wiki/Cell_biology of
which 30 pages covering cell structure and function have to be read prior to
working through the learning activities to follow. the Guide. Focus your attention
mainly to Chapter 2 of the Study Guide as it applies to the structure and function
of ells only.
3. Prokaryotes:
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html
4. Cellular organisation:
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookCELL2.html
Abstract: The introductory section address the differentiation of cells with specific
reference to cell structure and its relationship to cell division (mitosis and meiosis)
and the transfer of genetic material.
Rationale: The main aim with the readings is to allow you to come to terms with
the structures of prokaryotic and eukaryotic cells and also to relate the structure of
the cells to function with specific reference to mitosis, meiosis and the transfer of
genetic material.
Reading 2: Mitochondrion - From Wikipedia, the free encyclopaedia
Complete reference: http://en.wikipedia.org/wiki/Mitochondria
(Downloaded 28th August 2006)
Abstract: The chapter commences describing the structure of a mitochondrion,
followed by energy conversion and the release of great amounts of heat. The
chapter also links to a later section of the work where the task of the
mitochondrion in terms of genetic transfer and population genetic studies will be
highlighted. What the chapter also traces is to explain how mitochondrial
inheritance occurs and what influence this could have on future generations.
Rationale: The main purpose with this section of the work is to give you the
opportunity in working through an on-line text with the main intention of
acquainting yourself with the basic structure and function of mitochondria. The text
is vividly illustrated containing a variety of on-line links that will give you access to
the discussion and description of all the minute details relating to cell
mitochondria.
Reading 3: Cell organelles
Complete reference: http://en.wikipedia.org/wiki/organelles
(Downloaded 28th August 2006)
Abstract: The following description applies specifically to the second ‘Wikipedia’
references listed as URL in this section. It mentions the fact that eukaryotes are
the most structurally complex known cell type, and by definition are in part
organized by smaller interior compartments, that are themselves enclosed by lipid
membranes that resemble the outer most cell wall. The larger organelles, such as
the nucleus and vacuoles, are easily visible with moderate magnification (although
sometimes a clear view requires the application of chemicals that selectively stain
parts of the cells); they were among the first biological discoveries made after the
invention of the microscope. The article continues to explain that not all eukaryotic
cells have all of the organelles listed and occasionally, exceptional species of cells
are missing organelles that might otherwise be considered universal to eukaryotic
cells (such as mitochondria). There are also occasional exceptions to the number
of membranes surrounding organelles.
Rationale: We have included this article to your reading resources as it could be
regarded as a very comprehensive reader illustrating and explaining the structures
and function of the majority of organelles contained in prokaryotic and eukaryotic
cells. The different organelles are thoroughly compared with vivid and very clear
descriptions relating to structure and function. It is once again a very good text to
work through and to be reminded of the major differences of prokaryotic and
eukaryotic cells.
Reading 4: Cell Membranes Tutorial (*optional)
Complete reference:
http://www.biology.arizona.edu/cell_bio/problem_sets/membranes/index.html
Abstract: As explained in the above-mentioned website “this exercise introduces
the dynamic complexes of proteins, carbohydrates, and lipids that comprise cell
membranes. You should learn that membranes are fluid, with components that
move, change, and perform vital physiological roles as they allow cells to
communicate with each other and their environment. We also show that
membranes also are important for regulating ion and molecular traffic flow
between cells and that defects in membrane components, lead to many significant
diseases.”
The website suggests that you follow the following instructions:
“The following problems have multiple choice answers. Correct answers are
reinforced with a brief explanation. Incorrect answers are linked to tutorials to help
solve the problem.”
Rationale: The completion of a simple hands-on assessment activity will assist you
in mastering the cell membranes more effectively.
Reading 5: Cell nucleus
Complete reference: http://en.wikipedia.org/wiki/Cell_nucleus
Abstract: The following paragraph was taken from the following website:
http://en.wikipedia.org/wiki/Cell_nucleus. “The article illustrates that in cell biology
the nucleus is found in all eukaryotic cells and contains the nuclear genes that
form most of the cell's genetic material. The section explains that nuclei have two
primary functions namely to control chemical reactions within the cytoplasm and
also to store information needed for cellular division. Aside from containing the
cell's genome, the nucleus contains certain proteins whose interplay is thought to
regulate the expression of genes. Gene expression at the nuclear level involves
complex processes of transcription, pre-mRNA processing and the export of the
mature mRNA to the cytoplasm.”
Rationale: The section will provide you with a very thorough briefing on the
structure of the components of the cell nucleus in the first place, but will also
highlight the different functions of the nucleus as it applies to cell metabolism and
genetics.
Reading 6: Introduction to genetics
1. Complete reference: Genetics/Introduction
From Wikibooks, the open-content textbooks collection
"http://en.wikibooks.org/wiki/Genetics/Introduction"
2. Online Biology Book
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookTOC
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookgenintro.html
Abstract: The short article illustrates that genetics is the study of the function and
behaviour of genes and that offspring receive a mixture of genetic information from
both parents. This process contributes to the great variation of traits that we see in
nature, such as the colour of a flower’s petals, the markings on a butterfly’s wings,
or such human behavioural traits as personality or musical talent. Geneticists also
seek to understand how the information encoded in genes is used and controlled
by cells and how it is transmitted from one generation to the next. They also look
hw tiny variations in genes can disrupt an organism’s development or cause
disease. The article also explains how modern genetics involves genetic
engineering, a technique used by scientists to manipulate genes.
Rationale: Understanding the rationale behind genetics as field of study, will help
us to come to terms with the changes often experienced in genetic material, the
loci where these changes are actually taking place as well as the mechanism
behind the transfer of fixed characteristics from one generation to the other. The
examples provided in these introductory learning activities should prepare you to
comprehend the more advanced descriptions and activities to follow.
Reading 7: Fundamental understanding of Mendel’s law of dominance
1. Complete reference: Genetics/Mendelian Inheritance
http://en.wikibooks.org/w/index.php?title=Genetics/Mendelian_Inheritence&action=
edit&section=1
2. Online Biology Book
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookTOC
Abstract: The short article explains that Mendel's first step in the many
experiments he conducted was breeding pure breeding strains of peas. The traits
(traits = characteristics) he studied included pea colour, height and whether the
peas were wrinkled or smooth.
Mendel crossed the pure breeding Parental Generation (designated P). He found
that the first generation (F1) was exclusively phenotypically (phenotype =
externally visible characteristic such as pea colour) one of the parental types.
Mendel then crossed his F1 generation with itself. He found that the F2 generation
showed a surprising trait, three quarters were like the F1 generation, while the
remaining quarter were like the other Parents. From this Mendel realised that there
were two versions of each loci, one of which expressed dominance over the other.
He called this bi-particulate (bi = two) Inheritance. If a gene was following this 3:1
pattern it was said to be segregating normally.
Rationale: The purpose of allowing you to work through this introductory passage
on inheritance (with specific reference to the P1 and F2 generations) is to illustrate
to you how the simple crossing of two individuals containing pure traits such as a
pure breeding characteristic for shortness and a pure breeding characteristic for
tallness would produce offspring reflecting only one of these characteristics in the
first generation, called the F1 generation. The examples contained in the text are
very clear and self-explanatory.
Reading 8: Mitosis
Complete reference:
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmito.html
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookTOC.html
and
Abstract: The second website (consulted 5 October 2006) listed above under (2)
explains the reading of this specific article as follows:
“Despite differences between prokaryotes and eukaryotes, there are several
common features in their cell division processes. Replication of the DNA must
occur. Segregation of the "original" and its "replica" follow. Cytokinesis ends the
cell division process. Whether the cell was eukaryotic or prokaryotic, these basic
events must occur.”
Rationale: The article has been selected as a very basic comparative study
between binary fission and mitoses. The article explains the two processes very
carefully and with simple examples. The article has been selected as introductory
passage for the other processes (meiosis) and activities to follow).
Reading 9: Meiosis
Complete reference:
(1) http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeiosis.html
(2) http://en.wikipedia.org/wiki/meiosis
Abstract: The Wikipedia website (consulted 27 September 2006) listed above
summarises meiosis as follows:
“Meiosis is the process that transforms one diploid cell into four haploid cells in
eukaryotes in order to redistribute the diploid cell's genome. Meiosis forms the
basis of sexual reproduction and can only occur in eukaryotes. In meiosis, the
diploid cell's genome, which is composed of ordered structures of coiled DNA
called chromosomes, is replicated once and separated twice, producing four sets
of haploid cells each containing half of the original cell's chromosomes. These
resultant haploid cells will fertilize with other haploid cells of the opposite gender to
form a diploid cell again. The cyclical process of separation by meiosis and genetic
recombination through fertilization is called the life cycle. The result is that the
offspring produced during germination after meiosis will have a slightly different
blueprint which has instructions for the cells to work, contained in the DNA. This
allows sexual reproduction to occur.”
Rationale: Understanding meiosis will assist you in coming to a better
understanding of genetic transfer and of the role chromosomes play during the
transfer of characteristics.
Reading 10: Genetic manipulation – terminator, Terminator Technology, and
Genetically modified organisms
Complete reference: http://en.wikipedia.org/wiki/Terminator_(genetics)
http://en.wikipedia.org/wiki/Terminator_Technology
http://en.wikipedia.org/wiki/Genetically_modified_organism
Abstract: A genetically modified organism (GMO) is an organism whose genetic
material has been altered using techniques in genetics generally known as
recombinant DNA technology. Recombinant DNA technology is the ability to
combine DNA molecules from different sources into the one molecule in a test
tube. Thus, the abilities or the phenotype of the organism, or the proteins it
produces, can be altered through the modification of its genes.
The term generally does not cover organisms whose genetic makeup has been
altered by conventional cross breeding or by "mutagenesis" breeding, as these
methods predate the discovery of the recombinant DNA techniques. Technically
speaking, however, such techniques are, by definition, genetic modification.
Rationale: Terminator Technology is the colloquial name given to proposed
methods for restricting the use of genetically modified plants by causing second
generation seeds to be sterile. The technology was under development by the
U.S. Department of Agriculture and Delta and Pine Land Company in the 1990s
and is not yet commercially available. Because some stakeholders expressed
concerns that this technology might lead to dependence for poor smallholder
farmers, Monsanto, an agricultural products company, pledged not to
commercialize the technology even if and when is becomes commercially
available.
Reading 11: Genetic manipulation – terminator
Complete reference:
http://www.gse.buffalo.edu/FAS/Bromley/classes/socprac/readings/Steinbrecher.htm
The Ecologist, Sept-Oct 1998 v28 n5 p276(4), Terminator Technology: the threat to
world food security. Ricarda A. Steinbrecher; Pat Roy Mooney. Author's Abstract:
COPYRIGHT 1998 The Ecologist (UK).
Abstract: According to Steinbrecher and Mooney (1998:276) in The Ecologist 28(5)
“Monsanto's latest flagship technology makes a nonsense of its claim that it seeks to
feed the worlds hungry. On the contrary, it threatens to undermine the very basis of
traditional agriculture - that of saying seeds from year to year. What's more, this
"gene cocktail" will increase the risk that new toxins and allergens will make their
way into the food chain”.
Rationale: Because of the relevant importance of genetic manipulation in
agriculture, we have decided to include this article dealing with the controversial
manipulation of agricultural products and the responses it has evoked the last few
years.
Reading(s) #1
Reading(s)
#1.1
Botany/Print version - Wikibooks, collection of open-content textbooks
http://en.wikibooks.org/wiki/Botany/Print_version
Botany/Print version
< Botany
Note: current version of this book can be found at http://en.wikibooks.org/wiki/Botany
Table of contents
Introduction
Introduction
<< Contents Page
Introduction to the Botany Study Guide
This Study Guide to the Science of Botany is a textbook at Wikibooks shelved under Biology and intended to
establish a course of study in the subject of Botany, utilizing articles provided in Wikipedia
(http://www.Wikipedia.org/) , with links to other relevant web sites and other Wikibooks as appropriate. In
some cases, portions of the text from Wikipedia articles have been used to materially develop introductory
text within the Guide.
For the new user, it need be pointed out that Wikipedia differs from a standard encyclopedia in two
important respects: 1) it is a hypertext document, and 2) it is open and editable, and therefore constantly
changing. For the student following this or any guide through Wikipedia to cover a specific subject, it is
recommended that each article (page) be read first in its entirety, before any hyperlinks are followed to other
topics or explanations. It is too easy, otherwise, to simply become lost in a maze of links, and miss the main
thrust of an article presented as an assignment from the Guide. Because Wikipedia is constantly changing
(and, it is believed, improving) the quality of each article encountered will be variable. Some articles are well
written and go to adequate depth, whereas others, lacking a proponent, are shallow and incomplete. Short or
sloppy looking articles may contain questionable facts. These short-comings should diminish with time, but
can be a problem for the student.
One clear advantage to using this Guide linked to a hypertext like Wikipedia is the "circular redundancy with
serendipity" factor that arises when an article is read and its hyperlinks followed; this factor can be a
powerful learning tool. The persistent reader is subjected to a fairly high degree of repetitive reading, often
presenting slightly differing perspectives on the same general topic, with the result that learning comes from
redundancy and seeing difficult concepts presented in more than one way. At the same time, some
hyperlinks lead down less relevant paths, bringing new and unanticipated knowledge. If, as a student, you
are truely interested in mastering the subject of botany, you must be prepared to read beyond the basic
assignments; in some cases, beyond Wikipedia to explore other, "outside" web sites.
1 of 23
23/11/06 15:00
It seems likely that the typical user of the Study Guide to the Science of Botany is not necessarily an active
student taking a course in botany at the highschool (AP) or college level, but a person with a strong interest
in plants—an amateur naturalist or a gardener. Therefore the guide must incorporate both the basic
biological and physiological aspects of plants as well as extensive taxonomy-based coverage of the diversity
of plants and related organisms. The amount of material now available on the web covering the latter subject
is becoming nothing short of phenomenal. In effect, one now has access to much of the world's plant
diversity, with photographs and descriptions, in many cases from web sites maintained by specialists. One
goal of the guide is to provide a systematics-based approach to capturing this kind of information, hopefully
giving the student a strong background in plant systematics. The importance of this approach is not that
everyone should become a taxonomist—or become more familiar with plant taxonomy, a specialized field of
botanical science with a relatively narrow following—but that appreciation for (and understanding of)
species diversity is most critical at this time in our earth's history marked by accelerated species extinctions
and destruction of native ecosystems by both human population expansion and man-induced spread of
non-native species.
The Study Guide to the Science of Botany includes two other "parallel" documents intended to enhance the
usefulness of the Guide. These could also be used separately or independently as source documents for a
beginning course in Botany. They are the Discussion pages and the Laboratory Exercise pages. Both are
explained in detail in the next Section titled How to use this Guide.
How to use
<< Contents Page
How to Use the Botany Study Guide
The purpose of the Study Guide to the Science of Botany is to weave—out of the information on Life
Science and especially Botany contained in Wikipedia—a course of study for the student or layman. It is
anticipated that this course will be either supplemental to instruction being received at a school or college, or
will be self-directed. In either case, the Guide is not a novel and should not be approached as one. A smooth
flow of dialogue is simply not possible and should not be anticipated. The Guide may be closer to the
sometimes disjointed notes generated by a student from a lecture or careful reading of a detailed textbook.
Within each subsection of a Chapter, introductory text is followed by one or more "reading assignments" of
the form:
Read Botany (Links need not be pursued at this time)
Following (that is clicking on) the link (to Wikipedia "Botany" in this case) will open an article intended to
provide the details of the Chapter subsection. Recommended articles should be read from top to bottom, and
then re-read following some or all of the links embedded in the article to other articles for expanded
elucidation or to clarify terms; that is, in most cases, completion of an "assignment" (recommended article)
includes at least some or all articles linked to the first. Obviously, it cannot be the case that all links are
followed to articles, whose links are then followed to articles, and so on until no new material is encountered.
It is likely there would be no quick end to such a pursuit. The amount of time spent wandering beyond the
original article is partly a personal matter of how much the reader is getting out of the foray than anything
else. Realize it is certainly possible to wander well off the subject at hand. As in the example above, notes are
provided with assignments giving some direction for pursuing links. An instruction NOT to follow links
simply means the additional material will be encountered later in the course of instruction, and going beyond
2 of 23
23/11/06 15:00
the assigned article may provide too much detail for a beginning student. The following example:
Read Science (The following links are included:)
Scientific Method
Philosophy of Science
specifies that two other links ARE part of the assignment. Other links encountered may be followed to
expand your knowledge or, as always, to aid in understanding of technical terms encountered. Hyperlinks
included with the text in the Guide are there simply for convenience, usually to topics somewhat peripheral
to the main one. In all cases, finding your way back to the Guide may become tricky, but we have to leave
this up to you to establish, beyond pointing out that your browser's Back button is intended for this purpose.
Discussion Questions
At the end of each subsection are posted one to several questions. In general, you will get more out of these
questions if you write out your answer on a piece of paper. You may wish to accomplish this on the re-read,
allowing each question to guide your quest for an answer. A discussion page for each chapter provides
answers to the questions posed. However, the questions are intended to be thought-provoking, and may not
have a single straight-forward answer. Answers on the discussion pages are also necessarily much longer
than would be expected of any one student; it is expected that each student answer will fit somewhere within
the broad discussion presented.
Laboratory Exercises
A natural sciences course laboratory unit is supposed to provide hands-on experience in exploring topics
raised in the text and lecture units. The best that a website can give towards this goal is a manual that is
liberally provided with pictures and diagrams. The student must provide the "hands on" from the
neighboring natural world. Fortunately, in botany, this is much easier to accomplish than in almost any other
field of science. Both the outdoors, the local market, and (if available) a botanical garden can be sources of
materials for study. Indeed, we may teach the structure of a pome using an apple in the hope that the student
will end up with a pear.
In using any of the Laboratory Exercises, it is always best to read through the entire module before actually
doing anything. Resist the temptation to view the material as an instruction manual to be followed in a
specific order. For one thing it is difficult to write a module that covers, at each step, all that the student
should know before proceeding on to the next step. The value of any exercise will be significantly enhanced
if you have a pretty good idea where it is going in advance.
General Navigation
The Study Guide is divided into Sections and Chapters which define the subject material of each module. At
the bottom of each text page (main text of a module), is a short version of the Table of Contents, allowing
the reader to jump between chapters within a Section. Here is an example of the "Wiki Contents Table" for
Section I:
Botany Study Guide ~ Wiki Contents Table
Section I
Chapter 1 - Introduction ~ Chapter 2 - Plant cells
3 of 23
23/11/06 15:00
Chapter 3 - Plant Tissues ~ Chapter 4 - Plant Organs
Chapter 5 - Plant Reproduction ~ Chapter 6 - Plant Morphology
Note that at the beginning of each module, links are provided to both the previous chapter and the suceeding
chapter, as well as to the main Table of Contents. Links to units associated with a module, as for example to a
Laboratory Exercise, appear near the end of the module
Final Note
As a final note, read the next Section and consider how you might make a contribution to the Guide.Botany
Both this Guide and all articles in Wikipedia are free content that can be added to or edited by anyone. It is
an opportunity for the user of these documents to contribute information, or even state given information
more clearly, simply by editing a page. As a student with a textbook and a lecturer (teacher), you may find
yourself in possession of useful facts, another point of view on existing facts, or a report you prepared of
exceptional quality. Any of these can be added to an appropriate page in this Guide or the Wikipedia.
However, this caution is strongly advised: Do not place into the Guide any text or pictures taken verbatim (or
close to verbatim) from a text book, web site, or other copyrighted source without permission of the copyright
holder. In general, this means, anything you submit should be your own work.
To learn how to edit or contribute material to this textbook, first read the introduction at: How to
Edit.
<< Contents Page |
Chapter 1 | Chapter 2 >>|
Chapter 1. Introduction to Botany
Botany as a Science
Botany is the branch of biology concerned with
the scientific study of plants. Traditionally,
botanists studied all organisms that were not
generally regarded as animal. However, advances
in our knowledge about the myriad forms of life,
especially microbes (viruses and bacteria), have
led to spinning off from Botany the specialized
field called Microbiology. Still, the microbes are
usually covered in introductory Botany courses,
although their status as neither animal nor plant is
firmly established.
The earth star fungus (Geastrum aff. welwitschii).
Click here to enlarge image
Read Biology (Links need not be pursued at this
time)
Read Botany (Links need not be pursued at this time)
and note that other texts encompassing topics in Biology are available (under development) at
4 of 23
23/11/06 15:00
WikiBooks as sources of reference.
Plants are living entities, and material presented within Biology will have relevence here, most particularly at
the cellular and subcellular levels of organization (Chapter 2). Both plants and animals deal with the same
problems of maintaining life on planet Earth — their approaches seem quite different, but the end result is
the same: continued existence in an organized state, as part of a universe whose tendency is towards greater
disorganization. Back on Earth, however, it is a fact that microbes, plants, and animals comprise a very
interdependent system. We divide them apart, because our minds work best that way. We categorize and
learn common features or properties of the categories. This approach is neither right nor wrong, but is
clearly efficient for our minds. Nonetheless, it is desirable to regularly step back and realize that the
boundaries between categories are often just constructs, and exceptions to our categories usually abound.
It was alluded to in the opening definition that Botany is a science. Just what makes Botany, or anything else
a science? It is important to acquire a grasp of the fundamentals of science itself to fully appreciate both how
botanical knowledge was gained as well as how it can be used. It is usually quickly disinteresting to acquire
facts simply for the sake of knowing. Humans do not just appreciate mountains because they are there, they
climb them because they are there!
Read Science (The following links are included:)
Scientific Method
Philosophy of Science
Read Why study science? (http://www.people.virginia.edu/~rjh9u/studysci.html)
Questions:
1-1. Do you think the scientific method is something only a scientist would use?
Living Systems
Biology is defined as the study of life, and Botany is that discipline within Biology concerned with the study
of living organisms called plants and with certain other living things that are not plants (but are not animals
either).
Defining 'Plant'
Like many words in common usage that apply to biological entities or concepts, the term plant is more
difficult to define than might be at first obvious. Although botanists describe a Kingdom Plantae, the
boundaries defining members of Plantae are more exclusive than our common concept of a "plant". We are
tempted to regard plant as meaning a multicellular, eukaryotic organism that generally does not have
sensory organs or voluntary motion and has, when complete, a root, stem, and leaves. However, botanically
only vascular plants have a root, stem, and leaves, and even some vascular plants, such as certain
carnivorous plants and duckweed, fall afoul of that definition. But to be fair, the vascular plants are the
plants we tend to encounter every day and that most people would readily regard as "plants".
A more significant point of departure between Plantae and plants occurs among the seaweeds. Technically,
only a relatively minor group of seaweeds (the chlorophytes or green algae) are members of the Kingdom
Plantae. The majority of seaweeds, like the kelps (very large brown algae from the Order Laminariales),
despite a superficial appearance of such, lack true stems, leaves, roots, and any kind of vascular systems as
found in higher plants. Thus, the kelps are not Plantae; but are they plants? Certainly if we regard the green
algae as plants, it is difficult to exclude the more prominent red and brown algae of our coastal waters.
5 of 23
23/11/06 15:00
Another, much broader definition for plant is that it refers to any organism that is
photoautotrophic—produces its own food from raw inorganic materials and sunlight. This is not an
unreasonable definition, and is one that focuses on the role plants typically play in an ecosystem. However,
there are photoautotrophs among the Prokaryotes, specifically photoautotrophic bacteria and cyanophytes.
The latter are sometimes called (for good reasons) blue-green algae. Then there arises the problem that many
people would consider that a mushroom is a plant; a mushroom is the fruiting body of a fungus (Kingdom
Fungi) and not photoautotrophic at all, but saprophytic. However, there are more than a few species of
flowering plants, fungi, and bacteria that are not autotrophic, but parasitic.
We cannot hope to offer a firm answer. The list of characteristics that separate the Plantae from the other
biological kingdoms provides at least a technical definition, but realize it is only a technical definition. The
problem this lack of precision or agreement in the definition of "plant" presents is one of understanding
statements, often encountered in Wikipedia (and other) articles, of the sort: ...xylem is one of the two
transport tissues of plants. In general it cannot be assumed this means all plants, algae through flowering
plants. It very probably does not include fungi or bacteria. Indeed, it is usually safest to assume the
discussion is about vascular plants (essentially the ferns, conifers, flowering plants, and a few others; see
discusion below on "General Terminology") unless stated differently (e.g., ...in vascular and non-vascular
plants this is such and such).
Plants as Organisms
Read Organism (Links need not be pursued at this time)
Read Plants (Links need not be pursued at this time)
The distinction between life and non-life is not as easily made as you might think. There exist intracellular
"parasites" that are progressively less alive in terms of being metabolically active:
Viruses — Viroids — Prions
Questions:
1-2. What things do all plants seem to have in common that would justify
our classifying them together in their own Kingdom?
1-3. When you catch a cold a virus has infected your body.
Why do you think there is reason to question whether the virus is living or not?
After all, if you took some ricinin (a plant poison), you would get very sick,
but no one would suggest the toxin were alive or that the plant had entered your body.
Plants and their Uses
There can be no disputing the fundamental significance of plants to the ecology of our planet.
Photosynthetic plants utilize energy arriving from the sun to create complex organic molecules from
inorganic substances, and by this process contribute oxygen to the atmosphere. Advanced animal life is very
much dependent upon this source of oxygen, as well as the organic molecules that form the basis of nearly
every food web on the planet. However, humans utilize plants in many ways, especially as sources of
pleasure, food, and material for shelter, clothing, and more. Consider here the role plants play in our
everyday lives and in our economy.
6 of 23
23/11/06 15:00
Read
Read
Read
Read
Horticulture (Links need not be pursued at this time)
Gardening (Links need not be pursued at this time)
Agriculture (Links need not be pursued at this time)
Silviculture (Links need not be pursued at this time)
There is a Wikibook COOKBOOK
There is a Wikibook on GARDENING
Introduction to Plant Classification
At the beginning of this chapter it was suggested that each of us categorizes information we encounter on a
daily basis. Our minds seem to want to find relationships between facts and observations, to erect mental bins
in which to place new items with previous "facts". This natural human process is the basis for prejudice, in as
much as "facts" categorized together can become strongly associated. But these are personal constructs. In
order for scientists of many races, speaking many languages, and coming from all manner of backgrounds
and experiences to work productively together to solve common problems, the objects with which they work
must be classified within a universally accepted framework.
The classification of living things is called systematics, or taxonomy, and ideally should reflect the
evolutionary history (phylogeny) of the different organisms.
Read The evolutionary timeline to gain an understanding of how evolutionary history has played out on the planet
earth, paying particular attention to entries in the table that describe the early evolution of life and the later evolution of
terrestrial plant life.
Taxonomy arranges organisms in groups called taxa, while systematics seeks clues to their relationships. The
dominant system of Scientific Classification is called Linnaean taxonomy, and includes classification ranks
as well as an organism naming convention called binomial nomenclature.
Traditionally, all living things were divided into five kingdoms:
Monera — Protista — Fungi — Plantae — Animalia
However, this five-kingdom system has been replaced by Carl Woese's three-domain system, which focuses
on phylogenic roots and comparison of DNA structures. The older approach utilized visual observation as
the basis of classification. The three domains reflect whether cells have nuclei (eukaryotic) or not
(prokayotic), as well as differences in cell membranes and cell walls.
Archaea — Eubacteria — Eukaryota
Read Binomial nomenclature
Read Scientific classification
Recall (and review as necessary) how these groupings relate to the sequence of events in the evolutionary
history of life as summariaed in Timeline of Evolution. You will return to the subject of Scientific
Classification to consider in much more detail the groups of organisms studied in Botany, beginning with
Chapter 7. First, however, we shall turn our attention to the structure and function of cells and eventually to
gain an understanding of plant structure (plant anatomy) and function (plant physiology).
General Terminology
In Section II of this text we will delve much deeper into "plant" systematics. But you should be aware of
some general terms related to classificatory schemes that are used regularly in discussing plants. You have
probably encountered these terms many times, although may not be aware of their exact definitions. For
7 of 23
23/11/06 15:00
example, much of the material in Section I of this textbook is biased towards flowering plants. That is, much
of the descriptive material here as well as at Wikipedia refers specifically to these. Flowering plants are
angiosperms; plants that have flowers and produce seeds, and comprise the majority of the plants we would
normally encounter in say a nursery if not on the street, field, or empty lot. Seed-bearing plants include
both the angiosperms and the gymnosperms, the latter now treated as a modern group called conifers. The
conifers are also common plants, especially in higher latitudes, but bear cones instead of flowers. Both
conifers and flowering plants develop vascular tissues internally that conduct fluids (especially water)
throughout the plant. Included in the vascular plants are ferns. Ferns have vascular tissue, but reproduce by
spores. They do not produce seeds and do not bear flowers.
Laboratory Excercises for Chapter 1 >>
Discussion of questions for Chapter 1 >>
Section I
<< Contents Page | << Chapter 1 |
Chapter 2. Plant Cells
Introduction
A cell is a very basic structure of all living systems, consisting of
protoplasm within a containing cell membrane. Only entities such as
viruses—literally on the boundary between non-living chemicals and
living systems—lack cells or basic cell structure. All plants, including
very simple plants called algae, and all animals are made up of cells,
and these are organized in various ways to create structure and function
in an organism. Biologists recognize two basic types of cells:
prokaryotic and eukaryotic. Prokaryotic cells are structurally more
simple. They are found only in single-celled and some simple,
multicellular organisms (all bacteria and some algae, which all belong
to Bacteria and Archaea domains). Eukaryotic cells are found in most
algae, all higher plants, fungi, and animals (Eukarya domain). Thus,
differences between these two cell types are critical to how an organism
is classified, and an important consideration in the evolutionary
sequence of life on the planet Earth.
A lupe (left) and a hand-lens (right) tools used by botanists in the field
Plant Cell Structure
Nearly all cells are too small to be seen with the unaided eye. As always there are some exceptions, but
8 of 23
23/11/06 15:00
generally magnification is required to detect a cellular structure. In plants, a good hand-lens or lupe (see
photo at right) will sometimes suffice, but in working with cells or observing how cells are organized to
form tissues and structures, a high power microscope is used.
Read about Cells ~ You may wish to follow some or all links to "Main articles" as these provide detail that may interest
you; or you might return to explore further should questions arise later in the course.
Read Plant cells and, at minimum, articles at the following links (but ignore, for now, the topic of Plant Cell Types):
Cell Wall
Protoplasm and cytoplasm
Vacuole
Ergastic substances
Plastids
Chloroplasts
Also note that the textbook, Cell Biology, is available at WikiBooks and can be used as a more detailed
reference. You should read the Introductory Chapter (all subsections) at this time.
Questions:
1. Can you think of reasons why macroscopic organisms are multicellular? (Macroscopic means large, in the
sense of "not microscopic")
Basic Cell Function
You should, by now, have a general appreciation for the complexity of cellular structure. Improvements in
microscopy, especially development of the Electron microscope, have revealed that cells are not merely
membranous sacks containing fluid of gel-like consistency. The degree of organization of the cytoplasm
into organelles and their membranes should have you convinced that much (perhaps most) of what is really
going on around you on this planet is occurring at a scale that is simply inaccessable to your eyes. And while
you cannot be expected to directly observe chemical reactions at a molecular scale, contemplate that you
cannot, even with powerful optics, directly observe most of the structure where these reactions are somehow
controlled to produce outcomes favorable to life—indeed, are life. Hopefully, as you acquire knowledge and
become a biologist—a botanist—you will learn to recognize the relevant phenomena by their macroscopic
expressions (that which you can readly observe with the unaided eye).
To appreciate basic cell function, it is necessary to first list the processes or outcomes that cells must
accomplish to further existence. More specialized functions will be discussed under plant cell structure, as
our interest must eventually focus on plants. For now, recall that in your reading you have already
encountered these several basic abilities of cells:
Metabolism involves taking in of raw material to use in building cell components and breaking down
of other molecules to provide energy for various growth processes; byproducts may be released.
Protein biosynthesis by transcription of DNA to RNA and then translation to protein, used in growth
or released for use elsewhere by the organism.
Reproduction by cell division.
Now explore each in turn. Think initially of a single-celled organism with no special abilities, only a "will" to
stay alive and perpetuate itself. Remember, the environment will not be kind. The cell must grow and
reproduce to counter the tendency of outside forces to breakdown molecular structure and destroy life. Then
consider the situation where a cell is part of a multicellular organism, and may be performning more limited
and specialized functions.
Read Cell metabolism (Follow links and read at least these articles):
9 of 23
23/11/06 15:00
Metabolic pathway (Links need not be followed)
Cell respiration (Follows all links)
Read Protein biosynthesis (Follow links as necessary to understand process and terminology; Also included is:)
Gene expression
Read Cell reproduction (The following links are included:)
Mitosis
Cell cycle
Questions:
1. Have you been able to discern a relationship between genes and basic cell function? If so, is this also
a basic cell function, and where do we list it?
Plant Cell Specializations
We will learn about the cells of algae and other organisms (e.g., bacteria and fungi) traditionally covered
within Botany in later chapters on those organsims (Chapters 5 - 7). Here, we concentrate on the cells of
plants.
The simplest type of plant cell is called a parenchyma cell and most of the basic metabolic and reproductive
processess of the plant occur in these cells. A term for parenchyma cells with chloroplasts, is chlorenchyma
cells. Other plant cell types that we shall be considering are:
Collenchyma ~ living cells with thickened walls for increased support
Sclerenchyma ~ lignified dead cells forming fibers for increased support
Epidermal ~ surface covering
Cork
Xylem tracheid ~ single long (up to 1 mm) thin cells for transporting water and support
Xylem vessel ~ cells form individual elements in an even longer (up to 1 meter in extreme cases) tube
for transporting water
Meristematic cells ~ growth
Read Parenchyma cell
Laboratory Exercises for Chapter 2 >>
Discussion of questions for Chapter 2 >>>
Section I
Chapter 3. Plant Tissues
10 of 23
23/11/06 15:00
Introduction
Plant Tissues
Most plant cells are specialized to a greater or lesser degree, and arranged together in tissues. A plant tissue
can be simple or complex depending upon whether it is composed of one or more than one type of cell. The
simplest tissue found in plants is called parenchyma. The cells are not very specialized, more or less rounded
or angular where packed together, and thin-walled. A type of parenchyma called chlorenchyma because the
cells contain chloroplasts forms tissue (usually in the leaves) responsible for most of the photosynthesis
occurring in the plant. Note that in simple tissues at least (tissues comprised mostly of one cell type), the
tissue name follows from the cell type. However, tissues may also have unique anatomical names related to
where in the plant they occur.
Meristems
The growth of a plant requires a source of undifferentiated cells located in places where growth is needed
and can be initiated to further the body plan (in comparison to animals, plants are rather open in this regard).
Some enlargement in size is always possible by elongation or enlargement of existing cells, or by existing
cells simply dividing. But differentiation of one cell type into another is only possible if the initial cell
(mother cell) is not very specialized. Tissues comprised of cells that remain undifferentiated and supply, by
their divisions, cells to form new tissues and organs, are called meristems. Meristem tissue occurs in places
that allow for a very orderly pattern of growth.
Read Meristems
Section I
Chapter 4. Plant Vegetative Organs
Introduction
As was noted in the previous chapter,
most plant cells are specialized to a
greater or lesser degree, and arranged
together in tissues. A tissue can be simple
or complex depending upon whether it is
composed of one or more than one type
of cell. Tissues are further arranged or
combined into organs that carry out life
11 of 23
23/11/06 15:00
functions of the organism. Plant organs include the
The parrot's feather, an aquatic plant (Myriophyllum aquaticum)
Click to enlarge picture
leaf, stem, root, and reproductive structures. The
first three are sometimes called the vegetative organs
and are the subject of exploration in this chapter. Reproductive organs will be covered in Chapter 5.
The relationships of the organs within a plant body to each other remains an unsettled subject within plant
morphology. The fundamental question is whether these are truly different structures, or just modifications
of one basic structure (Eames, 1936; Esau, 1965). The plant body is an integrated, functional unit, so the
division of a plant into organs is largely conceptual, providing a convenient way of approaching plant form
and function. A boundary between stem and leaf is particularly difficult to make, so botanists sometimes use
the word shoot to refer to the stem and its appendages (Esau, 1965).
The Leaf
The plant leaf is an organ whose shape promotes efficient gathering of light for photosynthesis, but the form
of the leaf must also be balanced against the fact that most of the loss of water a plant might suffer is going
to occur at its leaves. Leaves are extremely variable in details of size, shape, and adornments like hairs.
Read: Leaf (By now, the links you encounter in this article should be known to you; follow any link terms you do not
understand)
Although the leaves of most plants carry out the same very basic functions, there is nonetheless an amazing
variety of leaf sizes, shapes, margin types, forms of attachment, ornamentation (hairs), and even color.
Examine the Leaves (forms) page to learn the extensive terminology used to describe this variation. Consider
that there are functional reasons for the modifications from a "basic" type.
The Stem
The stem arises during development of the embryo as part of the hypocotyl-root axis, at the upper end of
which are one or more cotyledons and the shoot primordium.
Read: Stem
The Root
The root is the (typically) underground part of the plant axis specialized for both anchoring the plant and
absorbing water and minerals.
Read: Root (Follow any links for terms you do not understand and to gain a complete picture of root structural variation)
Be sure to read about and understand the meaning of each (at a minimum) of the following
terms: adventitious roots, endodermis, epidermis, gravitropism, root cap, root hair, stele,
taproot.
Most of the material you have read discusses the root organ as found in the angiosperms (flowering plants).
However, among the vascular plants, only Psilotales lack such an organ, having instead rhizomes that bear
hair-like absorbing structures called rhizoids (Eames, 1936 in Esau, 1965).
Questions:
12 of 23
23/11/06 15:00
4-1. At this point the conceptual differences between cell types, tissues, organs,
and organisms may be somewhat confusing. Using the leaf as an example, describe
this structure in a way that considers the cell types, tissues, and organs for
that part of the leaf where photosynthesis is concentrated.
Laboratory Excercises for Chapter 4 >>
Discussion of questions for Chapter 4 >>>
Section I
<< Contents Page | Chapter 4 |
Chapter 5 | Chapter 6 >>
Chapter 5. Plant Reproduction
Vegetative Reproduction
Vegetative reproduction is asexual
reproduction—other terms that apply are
vegetative propagation or vegetative
multiplication. Vegetative growth is
enlargement of the individual plant;
vegetative reproduction is any process that
results in new plant "individuals" without
production of seeds (see The Seed below) or
spores. It is both a natural process in many,
many species as well as one utilized or
encouraged by horticulturists and farmers to
obtain quantities of economically valuable
plants. In this respect, it is a form of cloning
that has been carried out by humankind for
thousands of years and by "plants" for
hundreds of millions of years.
Noni (Morinda citrifolia) flowers and fruit, Note stages of progressive
maturation shown from a cluster of flowers to an accessory fruit
Read Vegetative Reproduction (Follow all links)
Sexual Reproduction
The Flower
The flower is the reproductive organ of plants classified as angiosperms—that is, the flowering plants
comprising the Division Magnoliophyta. All plants have the means and corresponding structures for
13 of 23
23/11/06 15:00
reproducing sexually, and these other cases will be explored in later chapters. However, because flowering
plants are the most conspicuous plants in almost all terrestrial environments, we justifiably devote this
chapter to the flowering plants alone. You will learn how other plant groups (and non-plant groups, such as
fungi) reproduce sexually in Section II of the The Guide.
The basic function of a flower is to produce seeds through sexual reproduction. Seeds are the next
generation, and serve as the primary method in most plants by which individuals of the species are dispersed
across the landscape. Actual dispersal is, in most species, a function of the fruit: structural parts that typically
surround the seed. But the seed contains the germ of life of the next generation.
Read Plant sexuality (Follow links you find interesting, concentrating on acquiring a grasp of the terminology)
Read The Flower (Follow links you find interesting, but at minimum read each of the following articles)
Read calyx - the sepals
Read corolla - the petals
Read androecium - the stamens
Read gynoecium - the pistil(s)
Be sure to read about and understand the meaning of each of the following terms: androecium,
anthesis, calyx, carpel, corolla, gynoecium, inferior ovary, nectary, perigynous, petal, pistil,
pollen, sepal, stamen, superior ovary, syncarpous.
Read Inflorescence
Be sure to read about and understand the meaning of each of the following terms: bract,
inflorescence, panicle, raceme, spadix, spikelet.
Questions:
5-1. Do you think the flower structure is in any way responsible for the considerable
success of flowering plants in populating the earth?
The Seed and Germination
the primary purpose of the seed is one of preserving the continuity of life—starting a new generation in a
new physical location. For large plants (shrubs and trees), this can be especially important because successful
germination and growth close to the parent may be difficult or impossible; the established plant monopolizes
light and water resources in its immediate vicinity. Seeds can also serve the function of overwintering or
surviving harsh conditions. The entire generation—every individual—may die in the Fall or the dry season.
In many annual species, only the seed exists during unfavorable dry or cold conditions.
The Seed (Follow all links on anatomy and function)
Read Germination
The Fruit
The fruit is the actual agent of dispersal in most flowering plants.
The Fruit
Laboratory Excercises (flowers) for Chapter 5 >>
Laboratory Excercises (seeds) for Chapter 5 >>
Discussion of questions for Chapter 5 >>
14 of 23
23/11/06 15:00
Section I
Chapter 5. Plant Reproduction Laboratory ~ Flowers
An orchid flower
This first laboratory excercise for Chapter
4 deals with the flowers of a ground
orchid from Southeast Asia. The
photograph on the right demonstrates the
descriptive terminology that can be
applied to this species. You may wish to
read about orchids to place this plant
taxonomically and better understand
unusual aspects of the structure of this
flower. In reading the description below,
be sure you understand how or why each
bolded word applies to this specimen.
Also, observe that the
flowering-through-fruiting sequence is
well demonstrated in the photograph
because each flower is in a slightly
different phase of its life cycle from bud
to fruit.
Spathoglottis plicata Blume —
The flowers of this orchid are
carried on an erect raceme
Infloresence of the orchid, Spathoglottis plicata (enlarge to examine).
growing out of the pseudobulb,
each flower subtended by a green
to purplish bract that becomes strongly reflexed with age. The purple sepals and petals are similar
and spreading, elliptic to elliptic-ovate; the labellum is in three distinct parts: the lateral lobes narrow
and erect, the middle lobe horizontal and cleft or 2-lobed. Lying above the latter is the narrowly
clavate column. The inferior ovary in Spathoglottis develops into a cylindrical capsule (fruit) as the
perianth withers.
4-1. Review the photograph of the inflorescence of the orchid. Which one of these statements is true:
a) this inflorescence demonstrates determinate growth
b) this inflorescence could as well be called a spike
c) the uppermost flower shows anthesis.
d) there are five petals, therefore this is a dicot.
Following are a series of photographs of flowers from
various plants. Note that by clicking on the word
15 of 23
23/11/06 15:00
"Examine" in each title, you can enlarge the particular photograph for
closer examination. Read each question and the offered answers carefully.
All parts of answer choice must be correct for that choice to be correct.
Photograph 1: Bidens torta (Examine)
4-2. The structure at B is:
a) leaf
b) corolla
c) ligule
d) sepal
e) petiole
4-3. Although the flowers in these three
photographs appear very different, the following
parts or floral structures are essentially the same:
a) AA and F
b) BB and C
c) G and H
d) F and B
e) D and BB
4-4. Which statement of the following applies to
the structure indicated at E :
Photograph 2. Hibiscus (Examine)
a) Pollen grains have landed on this pistil
b) Androecium of a monoecious plant
c) This is a spathe
d) E is an anther releasing pollen
e) This flower head is on a dioecious plant
<< Return to Chapter 5
Answers to Chapter 4 Laboratory Questions:
4-1 ~ c (this flower alone is capable of pollination)
4-2 ~ b (A flower head with tubular disk corollas)
4-3 ~ d (both F and B indicate petals of their respective
flowers)
4-4 ~ d (the androecium is supported on a tubular
structure (G) that surrounds the pistil (H)
Chapter 5. Plant Reproduction
Laboratory ~ Seeds
A monocot seedling
16 of 23
23/11/06 15:00
Photograph 3. Xanthosoma or 'Ape (Examine)
The photograph at the right shows two
germinating seeds. These are seeds of
the fan palm, Pritchardia remota, a
species found naturally only on
(endemic to) the remote island of Nihoa
in the Hawaiian islands. The seed on the
left is in the proper orientation in the
planting medium (only a part of the
fruit coat covering the seed is visible),
while the one on the right (in a slightly
earlier stage) has been laid on the
surface in order to better reveal
development of the root. The following
structures are labeled:
co - coleoptile or shoot pole of
plant axis; essentially a cap within
which the plumule (first leaf) is
developing, to eventually project
through as in the seedling on the
left.
ra - radicle; primordial root or root pole of axis (note tiny root cap), buried in the medium in the
seedling on the left.
sc - scuttelum; that part of the cotyledon that remains inside the seed to absorb food stored in an
endosperm.
(enlarge to examine).
<< Return to Chapter 5
License
GNU Free Documentation License
Version 1.2, November 2002
Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in
the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without
modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author
17 of 23
23/11/06 15:00
and publisher a way to get credit for their work, while not being considered responsible for modifications
made by others.
This License is a kind of "copyleft", which means that derivative works of the document must themselves be
free in the same sense. It complements the GNU General Public License, which is a copyleft license designed
for free software.
We have designed this License in order to use it for manuals for free software, because free software needs
free documentation: a free program should come with manuals providing the same freedoms that the
software does. But this License is not limited to software manuals; it can be used for any textual work,
regardless of subject matter or whether it is published as a printed book. We recommend this License
principally for works whose purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a notice placed by the
copyright holder saying it can be distributed under the terms of this License. Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein.
The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is
addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring
permission under copyright law.
A "Modified Version" of the Document means any work containing the Document or a portion of it, either
copied verbatim, or with modifications and/or translated into another language.
A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively
with the relationship of the publishers or authors of the Document to the Document's overall subject (or to
related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the
Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The
relationship could be a matter of historical connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding them.
The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of
Invariant Sections, in the notice that says that the Document is released under this License. If a section does
not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document
may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are
none.
The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover
Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at
most 5 words, and a Back-Cover Text may be at most 25 words.
A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose
specification is available to the general public, that is suitable for revising the document straightforwardly
with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some
widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to
a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent
modification by readers is not Transparent. An image format is not Transparent if used for any substantial
amount of text. A copy that is not "Transparent" is called "Opaque".
18 of 23
23/11/06 15:00
Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input
format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming
simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats
include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by
proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally
available, and the machine-generated HTML, PostScript or PDF produced by some word processors for
output purposes only.
The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to
hold, legibly, the material this License requires to appear in the title page. For works in formats which do not
have any title page as such, "Title Page" means the text near the most prominent appearance of the work's
title, preceding the beginning of the body of the text.
A section "Entitled XYZ" means a named subunit of the Document whose title either is precisely XYZ or
contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a
specific section name mentioned below, such as "Acknowledgements", "Dedications", "Endorsements", or
"History".) To "Preserve the Title" of such a section when you modify the Document means that it remains a
section "Entitled XYZ" according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to
the Document. These Warranty Disclaimers are considered to be included by reference in this License, but
only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is
void and has no effect on the meaning of this License.
2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncommercially,
provided that this License, the copyright notices, and the license notice saying this License applies to the
Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this
License. You may not use technical measures to obstruct or control the reading or further copying of the
copies you make or distribute. However, you may accept compensation in exchange for copies. If you
distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
3. COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed covers) of the Document,
numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover,
and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the
publisher of these copies. The front cover must present the full title with all words of the title equally
prominent and visible. You may add other material on the covers in addition. Copying with changes limited
to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as
verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as
many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either
19 of 23
23/11/06 15:00
include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque
copy a computer-network location from which the general network-using public has access to download
using public-standard network protocols a complete Transparent copy of the Document, free of added
material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of
Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an Opaque copy (directly or through your
agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any
large number of copies, to give them a chance to provide you with an updated version of the Document.
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3
above, provided that you release the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution and modification of the Modified
Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and
from those of previous versions (which should, if there were any, be listed in the History section of
the Document). You may use the same title as a previous version if the original publisher of that
version gives permission.
B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the
modifications in the Modified Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five), unless they release you from this
requirement.
C. State on the Title page the name of the publisher of the Modified Version, as the publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
F. Include, immediately after the copyright notices, a license notice giving the public permission to
use the Modified Version under the terms of this License, in the form shown in the Addendum below.
G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in
the Document's license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled "History", Preserve its Title, and add to it an item stating at least the
title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is
no section Entitled "History" in the Document, create one stating the title, year, authors, and publisher
of the Document as given on its Title Page, then add an item describing the Modified Version as stated
in the previous sentence.
J. Preserve the network location, if any, given in the Document for public access to a Transparent
copy of the Document, and likewise the network locations given in the Document for previous
versions it was based on. These may be placed in the "History" section. You may omit a network
location for a work that was published at least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.
K. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section,
and preserve in the section all the substance and tone of each of the contributor acknowledgements
and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles.
Section numbers or the equivalent are not considered part of the section titles.
M. Delete any section Entitled "Endorsements". Such a section may not be included in the Modified
Version.
N. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in title with any
Invariant Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections
20 of 23
23/11/06 15:00
and contain no material copied from the Document, you may at your option designate some or all of these
sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's
license notice. These titles must be distinct from any other section titles.
You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your
Modified Version by various parties--for example, statements of peer review or that the text has been
approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a
Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any
one entity. If the Document already includes a cover text for the same cover, previously added by you or by
arrangement made by the same entity you are acting on behalf of, you may not add another; but you may
replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names
for publicity for or to assert or imply endorsement of any Modified Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under the terms defined
in section 4 above for modified versions, provided that you include in the combination all of the Invariant
Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections
may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different
contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of
the original author or publisher of that section if known, or else a unique number. Make the same adjustment
to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled "History" in the various original documents,
forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and
any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements."
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under this License,
and replace the individual copies of this License in the various documents with a single copy that is included
in the collection, provided that you follow the rules of this License for verbatim copying of each of the
documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License,
provided you insert a copy of this License into the extracted document, and follow this License in all other
respects regarding verbatim copying of that document.
7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent documents or works,
in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting
21 of 23
23/11/06 15:00
from the compilation is not used to limit the legal rights of the compilation's users beyond what the
individual works permit. When the Document is included in an aggregate, this License does not apply to the
other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the
Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers
that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in
electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the Document under
the terms of section 4. Replacing Invariant Sections with translations requires special permission from their
copyright holders, but you may include translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a translation of this License, and all the
license notices in the Document, and any Warranty Disclaimers, provided that you also include the original
English version of this License and the original versions of those notices and disclaimers. In case of a
disagreement between the translation and the original version of this License or a notice or disclaimer, the
original version will prevail.
If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement
(section 4) to Preserve its Title (section 1) will typically require changing the actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under
this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License. However, parties who have received copies, or rights,
from you under this License will not have their licenses terminated so long as such parties remain in full
compliance.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License
from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail
to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a
particular numbered version of this License "or any later version" applies to it, you have the option of
following the terms and conditions either of that specified version or of any later version that has been
published (not as a draft) by the Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not as a draft) by the Free Software
Foundation.
Retrieved from "http://en.wikibooks.org/wiki/Botany/Print_version"
Categories: Books with print version | Botany
22 of 23
23/11/06 15:00
This page was last modified 15:12, 17 June 2006.
All text is available under the terms of the GNU Free Documentation License (see Copyrights for
details).
Wikibooks® is a registered trademark of the Wikimedia Foundation, Inc.
Privacy policy
About Wikibooks
Disclaimers
23 of 23
23/11/06 15:00
Reading(s)
#1.2
Cell Biology/Print version - Wikibooks, collection of open-content t...
http://en.wikibooks.org/wiki/Cell_Biology/Print_version
Cell Biology/Print version
< Cell Biology
Note: current version of this book can be found at http://en.wikibooks.org/wiki/Cell_Biology
Remember to click "refresh" to view this version.
Table of contents
Introduction
Size of cells
What is a cell?
What is the difference between elements?
What is living?
What is interesting about cell biology?
Types of cells
Prokaryotes
Bacteria
Eukaryotes
Unique Properties of Plant Cells
Parts of the cell
Membranes
Organelles
Genetic material
Energy supply (chloroplasts and mitochondria)
Cell division
Cell cycle
Meiosis
Mitosis
Genes
Expression
Translation
1 of 32
23/11/06 15:03
Introduction
Size of cells
Cell Biology | Introduction
Size of cells | What is a cell >>
Size of Cells
Although it is generally the case that
biological cells are too small to be seen at
all without a microscope, there are
exceptions as well as considerable range in
the sizes of various cell types. Eukaryotic
cells are typically 10 times the size of
prokaryotic cells (these cell types are
discussed in the next Chapter). Plant cells
are on average some of the largest cells,
probably because in many plant cells the
inside is mostly a water filled vacuole.
So, you ask, what are the relative sizes of
biological
molecules
and cells?
The
following are all approximations:
2 of 32
Cells are so small that even a cluster of these cells
from a mouse only measures 50 microns
23/11/06 15:03
0.1 nm (nanometer) diameter of a hydrogen atom
0.8 nm Amino Acid
2 nm Diameter of a DNA Alpha helix
4 nm Globular Protein
6 nm microfilaments
10 nm thickness cell membranes
11 nm Ribosome
25 nm Microtubule
50 nm Nuclear pore
100 nm Large Virus
150-250 nm small bacteria such as Mycoplasma
200 nm Centriole
200 nm (200 to 500 nm) Lysosomes
200 nm (200 to 500 nm) Peroxisomes
800 nm giant virus Mimivirus
1 µm (micrometer)
(1 - 10 µm) the general sizes for Prokaryotes
1 µm Diameter of human nerve cell process
2 µm E.coli - a bacterium
3 µm Mitochondrion
5 µm length of chloroplast
6 µm (3 - 10 micrometers) the Nucleus
9 µm Human red blood cell
10 µm
(10 - 30 µm) Most Eukaryotic animal cells
(10 - 100 µm) Most Eukaryotic plant cells
90 µm small Amoeba
100 µm Human Egg
up to 160 µm Megakaryocyte
up to 500 µm giant bacterium Thiomargarita
up to 800 µm large Amoeba
1 mm (1 millimeter, 1/10th cm)
1 mm Diameter of the squid giant nerve cell
120 mm Diameter of an ostrich egg (a dinosaur egg was much larger)
3 meters Length of a nerve cell of giraffe's neck
Related reading
Some early history related to the development of an understanding of the
existence and importance of cells. The importance of microscopy.
What limits cell sizes?
Prokaryotes - Limited by efficient metabolism
Animal Cells (Eukaryotic) - Limited by Surface Area to Volume ratio
Plant Cells (Eukaryotic) - Have large sizes due to large central vacuole which is
responsible for their growth
Size of cells | What is a cell >>
Cell Biology | Introduction
|What is a cell?
Cell biology | Introduction
<< Size of cells | What is a cell | What is the difference between elements >>
3 of 32
23/11/06 15:03
Cells are the fundamental building blocks of life. Cells vary to form individual
"single-cell" organisms (bacteria) to "multi-cellular" structures (tissue, organs) and
organisms (animals and plants).
Cells are structural units that make
up plants and animals; also, there are
many single celled organisms. What
all living cells have in common is that
they are small 'sacks' composed
mostly of water. The 'sacks' are
made from a phospholipid bilayer
membrane.
This
membrane
is
semi-permeable
(allowing
some
things to pass in or out of the cell
while blocking others). There exist
other methods of transport across
this membrane that we will get into
later.
Schematic of typical animal cell, showing subcellular
components. Organelles: (1) nucleolus (2) nucleus (3)
ribosome (4) vesicle (5) rough endoplasmic reticulum
(ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER
(9) mitochondria (10) vacuole (11) cytoplasm (12)
lysosome (13) centrioles
So what is in a cell? Cells are 90%
fluid (called cytoplasm) which consists
of
free
amino
acids,
proteins,
carbohydrates, fats, and numerous
other
molecules.
The
cell
environment (i.e., the contents of the cytoplasm and the nucleus, as well as the way
the DNA is packed) affect gene expression/regulation, and thus are VERY important
aspects of inheritance. Below are approximations of other components (each
component will be discussed in more detail later):
Elements
59% Hydrogen (H)
24% Oxygen (O)
11% Carbon (C)
4% Nitrogen (N)
2% Others - Phosphorus (P), Sulphur (S), etc.
Molecules
50%
15%
15%
10%
10%
protein
nucleic acid
carbohydrates
lipids
Other
Components of cytoplasm
Cytosol - contains mainly water and numerous molecules floating in it- all except
4 of 32
23/11/06 15:03
the organelles.
Organelles (which also have membranes) in 'higher' eukaryote organisms:
Nucleus (in eukaryotes) - where genetic material (DNA) is located, RNA is
transcribed.
Endoplasmic Reticulum (ER) - Important for protein synthesis. It is a
transport network for molecules destined for specific modifications and
locations. There are two types:
Rough ER - Has ribosomes, and tends to be more in 'sheets'.
Smooth ER - Does not have ribosomes and tends to be more of a
tubular network.
Ribosomes - half are on the Endoplasmic Reticulum, the other half are 'free'
in the cytosol, this is where the RNA goes for translation into proteins.
Golgi Apparatus - important for glycosylation, secretion. The Golgi
Apparatus is the "UPS" of the cell. Here, proteins and other molecules are
prepared for shipping outside of the cell.
Lysosomes - Digestive sacks found only in animal cells; the main point of
digestion.
Peroxisomes - Use oxygen to carry out catabolic reactions, in both plant
and animals. In this organelle, an enzyme called catalase is used to break
down hydrogen peroxide into water and oxygen gas.
Microtubules - made from tubulin, and make up centrioles,cilia,etc.
Cytoskeleton - Microtubules, actin and intermediate filaments.
Mitochondria - convert foods into usable energy. (ATP production) A
mitochondrion does this through aerobic respiration. They have 2
membranes, the inner membranes shapes differ between different types of
cells, but they form projections called cristae. The mitochondrion is about the
size of a bacteria, and it carries its own genetic material and ribosomes.
Vacuoles - More commonly associated with plants. Plants commonly have
large vacuoles.
Organelles found in plant cells and not in animal cells:
Plastids - membrane bound organelles used in storage and food production.
These are similar to entire prokaryotic cells - for example, like mitochondria
they contain their own DNA and self-replicate. They include:
Chloroplasts - convert light/food into usable energy. (ATP production)
Leucoplasts - store starch, proteins and lipids.
Chromoplasts - contain pigments. (E.g. providing colors to flowers)
Cell Wall - found in prokaryotic and plant cells; provides structural support
and protection.
<< Size of cells | What is a cell | What is the difference between elements >>
What
is
elements?
the
difference
between
5 of 32
23/11/06 15:03
<< What is a cell | What is the difference between elements | What is living >>
The various elements that make up the cell are:
59% Hydrogen (H)
24% Oxygen (O)
11% Carbon (C)
4% Nitrogen (N)
2% Others - Phosphorus (P), Sulphur (S), etc.
The difference between these elements is their respective atomic weights, electrons,
and in general their chemical properties. A given element can only have so many other
atoms attached. For instance carbon (C) has 4 electrons in its outer shell and thus can
only bind to 4 atoms; Hydrogen only has 1 electron and thus can only bind to one other
atom. An example would be Methane which is CH4. Oxygen only has 2 free electrons,
and will sometimes form a double bond with a single atom, which is an 'ester' in organic
chemistry (and is typically scented).
Methane
Water
H
|
H-C-H
|
H
Methanol (Methyl Alcohol)
H
|
H-C-O-H
|
H
H
H
\ /
O
As for the organic molecules that make up a typical cell:
50%
15%
15%
10%
10%
protein
nucleic acid
carbohydrates
lipids
Other
Here is a list of Elements, symbols, weights and biological roles.
6 of 32
Element
Symbol
Atomic
Weight
Biological Role
Calcium
Ca
40.1
Bone; muscle contraction, second messenger
Carbon
C
12.0
Constituent (backbone) of organic molecules
Chlorine
Cl
35.5
Digestion and photosynthesis
23/11/06 15:03
Copper
Cu
63.5
Part of Oxygen—carrying pigment of mollusk
blood.
Fluorine
F
19.0
For normal tooth enamel development
Hydrogen
H
1.0
Part of water and all organic molecules
Iodine
I
126.9
Part of thyroxine (a hormone)
Iron
Fe
55.8
Hemoglobin, oxygen
animals
Magnesium Mg
24.3
Part of chlorophyll, the photosynthetic pigment;
essential to some enzymes.
Manganese Mn
54.9
Essential to some enzyme actions.
Nitrogen
N
14.0
Constituent of all proteins and nucleic acids.
Oxygen
O
16.0
Respiration; part of water; and in nearly
organic molecules.
Phosphorus P
31.0
High energy bond in ATP.
Potassium
K
39.1
Generation of nerve impulses.
Selenium
Se
79.0
For the working of many enzymes.
Silicon
Si
28.1
Diatom shells; grass leaves.
Sodium
Na
23.0
Part of Salt; nerve conduction
Sulfur
S
32.1
Constituent of most proteins. Important in protein
structure: Sulfide bonds are strong.
Zinc
Zn
65.4
Essential to alcohol oxidizing enzyme.
caring pigment of many
all
<< What is a cell | What is the difference between elements | What is living >>
What is living?
<< What is the difference between elements | What is living | What is interesting about
cell biology >>
The question, "What is life?" has been one of many long discussions and the answer
may depend upon your initial definitions.
7 of 32
23/11/06 15:03
Some definitions of life are:
1. The quality that distinguishes a vital and functional being from a non-living or dead
body or purely chemical matter.
2. The state of a material complex or individual characterized by the capacity to
perform certain functional activities including metabolism, growth, and
reproduction.
3. The sequence of physical and mental experiences that make up the existence of
an individual.
Under these definitions life may or may not include a virus that is only 'alive' if it can
insert its genetic material into a living cell. To some, living systems that react to the
environment, grow, improve, and reproduce are alive. A more liberal definition would
include too much, a narrower one would not include all cells.
<< What is the difference between elements | What is living | What is interesting about
cell biology >>
What is interesting about cell biology?
<< What is living | What is interesting about cell biology
What makes Cell Biology particularly interesting is that there is so much that is not fully
understood. A cell is a complex system with thousands of molecular components
working together in a coordinated way to produce the the phenomenon we call "life".
During the 20th century these molecular components were identified (for example, see
Human Genome Project), but research continues on the details of cellular processes
like the control of cell division and cell differentiation. Disruption of the normal control
of cell division can cause abnormal cell behavior such as rapid tumor cell growth.
Cells have complex interactions with the surrounding environment. Whether it is the
external world of a single celled organism or the other cells of a multicellular organism,
a complex web of interactions is present. Study of the mechanisms by which cells
respond appropriately to their environments is a major part of cell biology research and
often such studies involve what is called signal transduction. For example, a hormone
such as insulin interacting with the surface of a cell can result in the altered behavior of
hundreds of molecular components inside the cells. This sort of complex and finely
tuned cell response to an external signal is required for normal metabolism and to
prevent metaboic disorders like Type II diabetes.
Most of the cells of a multi-cellular organism have the same genetic material in every
cell, yet, there are over 200 types of cells in the body that are different shapes, sizes
and and carry out very different functions. And ALL of these cells were developed from
one special cell, a zygote. The study of how the many cell types develop during
8 of 32
23/11/06 15:03
embryonic development (Developmental Biology) is a branch of Biology that is heavily
dependent on the use of microscopy. Much of the control of cell differentiation is at the
level of the control of gene transcription, the control of which mRNAs are made. Muscle
cells make muscle proteins and nerve cells make brain proteins. Geneticists, molecular
biologists and cell biologists are working to discover the details of how cells specialize
to accomplish hundreds of functions from muscle contraction to memory storage.
Summary
Complexity in:
inter-relations between cells
signal transduction pathways inside cells
control of cell death and cell reproduction
control of cell differentiation
control of cell metabolism.
<< What is living | What is interesting about cell biology
Types of cells
Prokaryotes
Biology Cell biology | Types of cells
Prokaryotes | Eukaryotes >>
9 of 32
23/11/06 15:03
Most of these prokaryotic cells are small,
ranging from 1 to 10 microns with a
diameter no greater than 1 micron. The
major differences between Prokaryotic and
Eukaryotic cells are that prokaryotes do not
have a nucleus as a distinct organelle and
rarely
have
any
membrane
bound
organelles
[mitochondria,
chloroplasts,
endoplasmic reticulum, golgi apparatus, a
cytoskeleton
of
microtubules
and
microfilaments] (the only exception may be
a bacterium discovered to have vacuoles).
Both types contain DNA as genetic material,
have a surrounding cell membrane, have
ribosomes[70
s],
accomplish
similar
functions, and are very diverse. For
instance, there are over 200 types of cells in
the human body, that vary greatly in size,
shape, and function.
The structures of two prokaryotic cells. The
Prokaryotes are cells without a distinct
bacterium (shown at the top) is a heterotrophs,
nucleus.They have genetic material but that
organisms that eat other organisms.
Cyanophytes are autotrophs, organisms that
material is not enclosed within a membrane.
make their food without eating other organisms.
Prokaryotes
include
bacteria
and
cyanophytes. The genetic material is a
single circular DNA strand and is located
within the cytoplasm. Recombination happens through transfers of plasmids (short
circles of DNA that pass from one bacterium to another). Prokaryoytes do not engulf
solids, nor do they have centrioles or asters. Prokaryotes have a cell wall made up of
peptidoglycan.
Prokaryotes | Eukaryotes >>
Biology_Cell_biology | Types of cells
Bacteria
Bacteria are prokaryotic, unicellular organisms. Bacteria are very small; so much so
that 1 billion could fit on 1 square centimeter of space on the human gums, and 1 gram
of digested food has 10 billion bacteria. Bacteria are the simplest living organisms.
Previously they fell under the Kingdom Moneran, but now they fall into two different
Kingdoms: Archaebacteria and Eubacteria. There are several differences between the
two.
Archaebacteria have no peptidoglycan in their cellular walls. They also have odd lipids
in their cell walls. Many are able to live in extreme places (like early Earth). There are
3 types of Archaebacteria. The first type is Methanogen. These use Carbon dioxide and
Hydrogen to make Methane. They are found in sewage, cows, and swamps, and they
10 of 32
23/11/06 15:03
do not take in oxygen. The second type is Extreme Halophile. These live in extremely
salty places (i.e.: the dead sea and great salt lake). Finally, the third type is
Thermoacidophiles. These prefer extremely hot, acidic areas (i.e.: hot springs and
volcanos).
Eubacteria have peptidoglycan in their cell walls, and they have no unusal lipids. They
have three shapes: bacilli (hot dog shaped), cocci (ball shaped), and spirilli (spring
shaped). Eubacteria can also have prefixes before their names: strepto, indicating
chains of the shaped bacteria, and straphylo, indicating clusters of the shaped bacteria.
Eubacteria are tested in labratories for Gram stains. Gram stains will determine if
antibiotics will work (Gram postive) or if they will not (Gram Negative). There are four
major types of Eubacteria: Cyanobacteria (green bacteria that infest fertilizer polluted
ponds and lakes and mass produce algae), Spirochetes (Gram negative bacteria on
which antibiotics do not work), Gram Positive (both gram positive that are used to
make yogurt, streptthroat is one of these), and Proteobacteria (E-coli). Bacteria also
have special structures: Plasmids (a small loop of DNA separate from the nuclear
region, which is used for creating genetic variety, inserting into other organisms, and
by genetic engineers) and Endospores (hard coat created by some bacteria in extreme
conditions--this is why canning jars must be boiled for a long time).
Reproduction is either through binary fusion (splitting of a cell with no variety in its
genes) or through several other forms that produce genetic variety: Transformation
(taking DNA from environment and incorparting it into themselves), Conjugation ("sex"
in which cilia hook together and the Plasmids exchange genes), and transduction (viri
infect the bacteria and the bacteria infects the virus with its Plasmid to move genes
throughout the population).
Bacteria produce poisons that can cause sickness: exotoxins, which are given off by the
Gram positive bacteria, and endotoxins, which are given off by Gram negative bacteria
as they die.
Eukaryotes
Cell biology
<< Prokaryotes | Eukaryotes | Plants >>
Eukaryotes are cells with a distinct nucleus, a structure in
which the genetic material (DNA) is contained, surrounded
by a membrane much like the outer cell membrane.
Eucaryotic cells are found in most algae, protozoa, all
multicellular organisms (plants and animals) including
humans. The genetic material in the nucleus forms multiple
chromosomes that are linear and complexed with proteins
that help the DNA 'pack' and are involved in regulation of
gene expression.
The cells of higher plants differ from animal cells in that
11 of 32
An animal Cell
23/11/06 15:03
they have large vacuoles, a cell wall, chloroplasts, and a lack of lysosomes, centrioles,
pseudopods, and flagella or cilia. Animal cells do not have the chloroplasts, and may or
may not have cilia, pseudopods or flagella, depending on the type of cell.
<< Prokaryotes | Eukaryotes | Plants >>
Cell biology
Unique Properties of Plant Cells
Biology_Cell_biology | Types of cells
<< Eukaryotes | Unique Properties of Plant Cells
Plant Cells have a number of important differences compared to their animal
counterparts. The major ones are the Chloroplasts, Cell walls and Vacuoles. Unlike
animal cells, plant cells do not have centrioles.
Chloroplasts
The chloroplasts are an organelle similar to the mitochondria in that they are self
reproducing and the are energy factories of the cell. There most of the similarities
ends. Chloroplasts capture light energy from the sun and convert it into ATP and sugar.
In this way the cell can support itself without food.
vvv
Vacuoles
Plants often have large structures containing water surrounded by a membrane in the
centre of their cells. These are vacuoles and act as a store of water and food (in
seeds), a place to dump wastes and a structural support for the cell to maintain turgor.
When the plant loses water the vacuoles quickly lose their water, and when plants have
a lot of water the vacuoles fill up. In mature plants there is usually one large vacuole in
the centre of the cell.
Cell walls
Plant cells are not flaccid like animal cells and have a rigid cell wall around them made
of fibrils of cellulose embedded in a matrix of several other kinds of polymers such as
pectin and lignin. The cellulose molecules are linear and provide the perfect shape for
intermolecular hydrogen bonding to produce long, stiff fibrils. It is the cell wall that is
primarily responsible for ensuring the cell does not burst in hypertonic surroundings.
This wiki is incomplete, you can help by expanding it
Nucleus: Is the most important organelle in the cell. It is in here one will find the genes
12 of 32
23/11/06 15:03
in the chromosomes .The genes controls all activities that carries out by all organisms
and this are passed on by the parents to their offspring to one generation to the next.
Cytoplasm: This cell has a very complex structure and can only be seen under a
microscope light, it consistence is a jelly like material between the outer membrane and
the inner nucleus.
Cell membrane: This membrane is made up of lipids or fats and proteins; it carries out
the function of protection, it protects what is inside the cell from the danger that is
present in the external environment.
Cell wall Structure produced by some cells outside their cell membrane; variously
composed of chitin, peptidoglycan, or cellulose. It is the cell wall that is primarily
responsible for ensuring the cell does not burst in hypertonic surroundings.
Chloroplast: This is a sack that contains pigments called chlorophyll. This subject
absorbs energy from the sun which the plant cell will use to combine carbon dioxide
and water to form glucose during photosynthesis. In this way the cell can support itself
without food.
Parts of the cell
Membranes
Cell biology | Parts of the cell
Membranes | Organelles >>
13 of 32
23/11/06 15:03
The phospholipid bilayer which
the cell membrane is an
example of, is composed of
various
cholesterol,
phospholipids, glycolipids and
proteins. Below is an example
of
a
simple
phospholipid
bilayer.
The smaller molecules shown
between the phospholipids are
Cholesterol molecules. They
help to provide rigidity or
stability to the membrane. The
two
main
components
of
phospholipids are shown in
these figures by blue circles
Plasma membrane bilayer
representing the hydrophilic
head groups and by long thin
lines representing the hydrophobic fatty acid tails.
Both the interior of the cell and the area surrounding the cell is made up of water or
similar aqueous solution. Consequently, phospholipids orient themsleves with respect to
the water and with each other so that the hydrophilic ("water loving") head groups are
grouped together and face the water, and the hydrophobic ("water fearing") tails turn
away from the water and toward each other. This self-organization of phospholipids
results in one of just a few easily recognizable structures. Cell membranes are
constructed of a phospholipid bilayer as shown above.
Smaller structures can also form, known as 'micelles' in which there is no inner layer of
of phospholipid. Instead, the interior of a micell is wholly hydrophobic, filled with the
fatty acid chains of the phospholipids and any other hydrophobic molecule they enclose.
Micelles are not so important for the understanding of cellular structure, but are useful
for demonstrating the principles of hydrophilicity and hydrophobicity, and for
contrasting with lipid bilayers.
At least 10 different types of lipids are commonly found in cell membranes. Each type
of cell or organelle will have a different percentage of each lipid, protein and
carbohydrate. The main types of lipids are:
Cholesterol
Glycolipids
Phosphatidylcholine
Sphingomyelin
Phosphatidylethnolamine
Phosphatydilinositol
14 of 32
23/11/06 15:03
Phosphatidylserine
Phosphatidylglycerol
Diphosphatidylglycerol (Cardiolipin)
Phosphatidic acid
1.
2.
3.
4.
5.
6.
Phospholipids
Cholesterol
Semi-permeability and Osmosis
Proteins and channels
Hydrophobicity
Self-assembly
Membranes | Organelles >>
Organelles
<< Membranes | Organelles | Genetic material >>
Nucleus
The nucleus contains genetic material
or DNA in the form of chromatin. It is
a double membraned stucture, with
pores on it. These pores act as a
"gateway" to help the nucleoplasm to
maintain
continuity
with
the
cytoplasm. The nucleus also contains
the nucleolus which is responsable
for
the
sythesis
(creation)
of
Ribosomes. The
nucleus is the
"thinking" center.
Mitochondria
Schematic of typical animal cell, showing subcellular
components. Organelles: (1) nucleolus (2) nucleus (3)
ribosome (4) vesicle (5) rough endoplasmic reticulum
(ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER
(9) mitochondria (10) vacuole (11) cytoplasm (12)
lysosome (13) centrioles
A mitochondrian is the organelle
responsible for a cell's metabolism. It
synthetizes ATP through a protein called ATP synthase. Mitochondria have a double
membrane. An outer membrane and a folded inner membrane. The internal
membrane, called the cristae is invaginated (folded or creased), to maximize surface
area enabling it to hold more ATP synthases.
15 of 32
23/11/06 15:03
Chloroplasts
Chloroplasts are found only in photosynthesizing cells; e.g. plant cells. Chloroplasts
carry out photosynthesis by using several photosystem proteins. Chloroplasts also give
a cell its green colour and are widely believed to have evolved from symbiotic
prokaryotes that adapted to live inside eukaryotic cells.
Ribosomes
Ribosomes are responsible for protein synthesis; they are composed of two subunits
that to elongate an aminoacid sequence.
Endoplasmic Reticulum
The Endoplasmic Reticulum (ER) acts as a transport from the nucleus and ribosomes to
the Golgi apparatus. There are two types of endoplasmic reticulum:
Smooth ER
Smooth ER act as transport for various things, mainly the RNA from the nucleus to the
ribosomes (RNA is a small piece of the DNA code specifically designed to tell the
ribosomes what to make). Smooth ER appears smooth in texture, hence the name.
Rough ER
Rough ER are "rough" because of the ribosomes embedded in them. The rough ER take
the protein to the Golgi apparatus to be packaged into vacuoles
Golgi Complex
The Golgi Complex bonds functional groups to different biomolecules to direct them to
their respective destinations. It basically "packages" the stuff into vacuoles. The Golgi
Complex looks like pieces of pita bread stacked on top of each other.They are the ones
that have their origin from the ER.They basically function as the delivery system of the
cell.
Vacuole
Vacuoles are storage places. They store water or cell waste products.
Central Vacuole
The central vacuole is found only in plant cells. It is filled with water and is pressurised,
like a balloon. This forces all the other organelles within the cell out toward the cell wall.
This pressure is called turgor pressure and is what gives plants their "crisp" and firm
structure.
16 of 32
23/11/06 15:03
Peroxisomes
Peroxisomes perform a variety of metabolic processes and as a by-product, produce
hydrogen peroxide. Peroxisomes use peroxase enzyme to break down this hydrogen
peroxide into water and oxygen.
Lysosomes
Lysosomes are vacuoles containing digestive and destructive membranes. In white
blood cells, these are used to kill the bacteria or virus, while in tadpole-tail cells they
kill the cell by separating the tail from the main body.
A day in the life of a cell
The nucleus has translated a piece of its DNA, called RNA, and sent it to the
ribosomes embedded in the rough endoplasmic reticulum. These ribosomes then
make some valuable protein, which is then transported to the Golgi Apparatus. The
Golgi Apparatus "packages" the protein into a secretory vesicle where it eventually
reaches the cell membrane ready for secretion out of the cell and into the external
environment.
Links
For more info go to http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/cells.htm
<< Membranes | Organelles | Genetic material >>
Genetic material
<< Organelles | Genetic material | Energy supply (chloroplasts and mitochondria) >>
1.
2.
3.
4.
5.
6.
7.
8.
17 of 32
Genetic material of Prokaryotes
Genetic material of Eukaryotes
Nucleus
Nuclear membrane
Nucleolus
Codons
RNA polymerase
Histones
23/11/06 15:03
<< Organelles | Genetic material | Energy supply (chloroplasts and mitochondria) >>
Energy
supply
mitochondria)
(chloroplasts
and
<< Genetic material | Energy supply (chloroplasts and mitochondria)
Chloroplasts are the organelles that incorporate energy into storage while mitochondria
are the ones that release the energy from the stores.
Chloroplasts are flat discs usually 2-10 micrometer in
diameter and 1 micrometer thick. The chloroplast has a two
membrane envelope termed the Inner & Outer membrane
respectively.
Between
these
two
layers
is
the
intermembrane space.
Photosynthesis
1. Light Dependent Reactions
2. Calvin-Benson Cycle
The inside of a chloroplast
with the granum circled.
Respiration
1. Glycolysis
2. Krebs cycle
3. Electron transport
<< Genetic material | Energy supply (chloroplasts and mitochondria)
Cell division
Cell cycle
Cell biology | Cell biology:Cell division
The normal cell cycle consists of 3 major stages. The first is Interphase, during which
18 of 32
23/11/06 15:03
the cell lives and grows larger. The second is Mitosis, when the cell divides. The final
one is Cytokinesis, which is when the two daughter cells complete their separation.
From Wikipedia
The cell cycle is the cycle of a biological cell, consisting of repeated mitotic cell division
and interphase (the growth phase). A cell spends the overwhelming majority of its time
in the growth phases.
Overview
The cell cycle consists of
G1
phase,
the
first
growth phase
S phase, during which
the DNA is replicated,
where S stands for the
Synthesis of DNA.
G2 phase is the second
growth phase, also the
preparation phase for
the
M phase or mitosis and
cytokinesis, the actual
division of the cell into
two daughter cells
The cell cycle stops at several
checkpoints and can only
proceed if certain conditions
Schematic of the cell cycle. I=Interphase, M=Mitosis.
are met, for example, if the
The duration of mitosis in relation to the other phases has
cell has reached a certain
been exaggerated in this diagram.
diameter. Some cells, such as
neurons,
never
divide
once
they
become
locked
in
a
G0
phase..........................................................v
Details of mitosis
19 of 32
23/11/06 15:03
Schematic of interphase (brown) and mitosis (yellow).
Meiosis
Meiosis is a special type of cell division that is designed to produce gametes. Like
normal cell division, the cell will be double diploid and have a pair of each chromosome.
Meiosis consists of 2 cell divisions, and results in four cells. The first division is when
genetic crossover occurs and the traits on the chromosomes are shuffled. The cell will
perform a normal prophase, then enter metaphase during which it begins the
crossover, then proceed normally through anaphase and telophase.
The first division produces two normal diploid cells, however the process is not
complete. The cell will prepare for another division and enter a second prophase.
During the second metaphase, the chromosome pairs are separated so that each new
cell will get half the normal genes. The cell division will continue thorough anaphase and
telophase, and the nuclei will reassemble. The result of the divisions will be 4 haploid
gamete cells.
From Wikipedia
Crossover
Crossover is the process by
which
two
chromosomes
paired up during prophase I
of meiosis exchange a distal
portion
of
their
DNA.
Crossover occurs when two
chromosomes, normally two
homologous instances of the
same
chromosome,
break
and connect to each other's
ends. If they break at the
same
locus,
this
merely
results in an exchange of
genes. This is the normal way in which crossover occurs. If they break at different loci,
the result is a duplication of genes on one chromosome and a deletion on the other. If
they break on opposite sides of the centromere, this results in one chromosome being
lost during cell division.
Any pair of homologous chromosomes may be expected to cross over three or four
times during meiosis. This aids evolution by increasing independent assortment, and
reducing the genetic linkage between genes on the same chromosome.
20 of 32
23/11/06 15:03
Mitosis
Mitosis is the normal type of cell division. Before the cells can divide, the chromosomes
will have duplicated and the cell will have twice the normal set of genes.
The first step of cell division is prophase, during which the nucleus dissolves and the
chromosomes begin migration to the midline of the cell. (Some biology textbooks insert
a phase called "prometaphase" at this point.)The second step, known as metaphase,
occurs when all the chromosomes are aligned in pairs along the midline of the cell. As
the cell enters anaphase, the chromatids, which form the chromosomes, will separate
and drift toward opposite poles of the cell. As the separated chromatids, now termed
chromosomes, reach the poles, the cell will enter telophase and nuclei will start to
reform. The process of mitosis ends after the nuclei have reformed and the cell
membrane begins to separate the cell into two daughter cells, during cytokinesis.
From Wikipedia
In biology, Mitosis is the process of
chromosome segregation and nuclear
division that follows replication of the
genetic material in eukaryotic cells.
This process assures that each
daughter nucleus receives a complete
Mitosis divides genetic information during cell division.
copy of the organism's genetic
material. In most eukaryotes, mitosis
is accompanied with cell division or cytokinesis, but there are many exceptions, for
instance among fungi. There is another process called meiosis, in which the daughter
nuclei receive half the chromosomes of the parent, which is involved in gamete
formation and other similar processes.
Mitosis is divided into several stages, with the remainder of the cell's growth cycle
considered interphase. Properly speaking, a typical cell cycle involves a series of
stages: G1, the first growth phase; S, where the genetic material is duplicated; G2, the
second growth phase; and M, where the nucleus divides through mitosis. Mitosis is
divided into prophase, prometaphase, metaphase, anaphase and telophase.
The whole procedure is very similar among most eukaryotes, with only minor
variations. As prokaryotes lack a nucleus and only have a single chromosome with no
centromere, they cannot be properly said to undergo mitosis.
Prophase
21 of 32
23/11/06 15:03
The genetic material (DNA), which normally exists in the form
of chromatin condenses into a highly ordered structure called
a chromosome. Since the genetic material has been
duplicated, there are two identical copies of each chromosome
in
the
cell.
Identical
chromosomes
(called
sister
chromosomes) are attached to each other at a DNA element
present on every chromosome called the centromere. When
chromosomes are paired up and attached, each individual
chromosome in the pair is called a chromatid, while the whole
unit (confusingly) is called a chromosome. Just to be even
more confusing, when the chromatids separate, they are no
longer called chromatids, but are called chromosomes again.
The task of mitosis is to assure that one copy of each sister
chromatid - and only one copy - goes to each daughter cell
after cell division.
Prophase: The two
round objects above the
nucleus are the
centrosomes. Note the
condensed chromatin.
The other important piece of hardware in mitosis is the centriole, which serves as a sort
of anchor. During prophase, the two centrioles - which replicate independently of
mitosis - begin recruiting microtubules (which may be thought of as cellular ropes or
poles) and forming a mitotic spindle between them. By increasing the length of the
spindle (growing the microtubules), the centrioles push apart to opposite ends of the
cell nucleus. It should be noted that many eukaryotes, for instance plants, lack
centrioles although the basic process is still similar.
Prometaphase
Some biology texts do not include this phase, considering it a
part of prophase. In this phase, the nuclear membrane
dissolves in some eukaryotes, reforming later once mitosis is
complete. This is called open mitosis, found in most
multicellular forms. Many protists undergo closed mitosis, in
which the nuclear membrane persists throughout.
Now kinetochores begin to form at the centromeres. This is a
complex structure that may be thought of as an 'eyelet' for the
microtubule 'rope' - it is the attaching point by which
chromosomes may be secured. The kinetochore is an
enormously complex structure that is not yet fully understood.
Two kinetochores form on each chromosome - one for each
chromatid.
When the spindle grows to sufficient length, the microtubules
begin searching for kinetochores to attach to.
Metaphase
22 of 32
Prometaphase: The
nuclear membrane has
degraded, and
microtubules have
invaded the nuclear
space. These
microtubules can attach
to kinetochores or they
can interact with
opposing microtubules.
23/11/06 15:03
As microtubules find and attach to kinetochores, they begin to
line up in the middle of the cell. Proper segragation requires
that every kinetochore be attached to a microtubule before
separation begins. It is thought that unattached kinetochores
control this process by generating a signal - the mitotic spindle
checkpoint - that tells the cell to wait before proceeding to
anaphase. There are many theories as to how this is
accomplished, some of them involving the generation of
tension when both microtubules are attached to the
kinetochore.
Metaphase: The
chromosomes have
aligned at the metaphase
plate.
When chromosomes are bivalently attached - when both
kinetochores are attached to microtubules emanating from
each centriole - they line up in the middle of the spindle,
forming what is called the metaphase plate. This does not
occur in every organism - in some cases chromosomes move back and forth between
the centrioles randomly, only roughly lining up along the midline.
Anaphase
Anaphase is the stage of meiosis or mitosis when
chromosomes separate and move to opposite poles of the
cell (opposite ends of the nuclear spindle). Centromeres are
broken and chromatids rip apart.
When every kinetochore is attached to a microtubule and the
chromosomes have lined up along the middle of the spindle,
the cell proceeds to anaphase. This is divided into two
phases. First, the proteins that bind the sister chromatids
together are cloven, allowing them to separate. They are
pulled apart by the microtubules, towards the respective
centrioles to which they are attached. Next, the spindle axis
elongates,
driving
the centrioles
(and the set of
chromosomes to which they are attached) apart to opposite
ends of the cell. These two stages are sometimes called
'early' and 'late' anaphase.
Early anaphase:
Kinetochore microtubules
shorten
At the end of anaphase, the cell has succeeded in separating identical copies of the
genetic material into two distinct populations.
Telophase
23 of 32
23/11/06 15:03
Now the nuclear membrane reforms around the genetic
material and the chromosomes are unfolded back into
chromatin. This is often followed by cytokinesis or cleavage,
where the cellular membrane pinches off between the two
newly separated nuclei, to form two new daughter cells.
Cytokinesis
Cytokinesis refers to the physical division of one eukaryotic
cell. Cytokinesis generally follows the replication of the cell's
Telophase: The pinching is
chromosomes,
usually
mitotically,
but
sometimes
known as the cleavage
meiotically. Except for some special cases, the amount of
furrow. Note the
decondensing chromosomes.
cytoplasm in each daughter cell is the same. In animal cells,
the cell membrane forms a cleavage furrow and pinches
apart like a balloon. In plant cells, a cell plate forms, which
becomes the new cell wall separating the daughters. Various patterns occur in other
groups.
Genes
Expression
Gene expression is the first stage of a process that decodes what the DNA holds in a
cell. It is the expression of a gene that gives rise to a protein.
How does gene expression occur?
It starts of with transcription that gives rise to the middlemen namely the RNA. The RNA
relay information from the chromosomal DNA to the cytoplasm where the machinary for
protein synthesis resides.
Translation occurs following transcription wherein the protein synthesis machinary gets
into action and uses its tools to read out the message that the RNA holds. The details of
this process are indeed very complex and will probably be dealt with in an advanced
writeup.
Translation
Note:This book needs to be de-wikified.
Keep in mind that most Wikibooks modules use inter-wiki linking
generally done much less than a typical Wikipedia article, and
terms that are critical to understanding the module. This means
most of the links can simply be removed altogether if content
Wikipedia during a transwiki move.
24 of 32
rather sparsely, and is
only to reference key
that for the most part
has been copied from
23/11/06 15:03
It is very seldom that you will find a Wikibook using a common word by itself, which is
also why most of the imported articles from Wikipedia are full of red links. It would be
better if you simply removed the red links altogether in most cases.
If you insist on keeping some or all of the links, please make sure that especially the
red links have been changed with a "wikipedia" prefix, such as [[wikipedia:Color|]]
or use the "w" abbrivation of [[w:Color|]]. Both are valid inter-wiki links on all
Wikimedia projects.
The Translation Phase of Genetic Expression is divided into 2 Steps Transcription and
Translation. During Transcription RNA Polymerase unzips the two halfs of the DNA
where it needs to transcript. Then free RNA bases Attach to the DNA bases with the
Polymerase starting at the promoter and ending at the Termination signal. From this the
RNA can become mRNA, rRNA, or tRNA. The mRNA is a ribbon like strand that takes the
genetic information from the nucleus of the cell to the ribosome. rRNA forms a globular
ball that attaches to the rough E.R. to help make ribosomes. finally the tRNA forms a
hair shaped landing base that reads the genetic information to make proteins.
Translation happens when mRNA is pulled through a ribosome and tRNA reads the RNA
bases on the mRNA to make anti-codons of 3 bases and brings amino-acids to form the
protein. This starts with the condon AUG and ends at UAG. When done the protein
forms the correct shape and does the task it was created for. This brings the genetic
code from the nucleus, which it never leaves, to the cytoplasm of the cell where
proteins are produced to upkeep the body.
License
GNU Free Documentation License
Version 1.2, November 2002
Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional and
useful document "free" in the sense of freedom: to assure everyone the effective
freedom to copy and redistribute it, with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the author and publisher a
way to get credit for their work, while not being considered responsible for
modifications made by others.
This License is a kind of "copyleft", which means that derivative works of the document
25 of 32
23/11/06 15:03
must themselves be free in the same sense. It complements the GNU General Public
License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because
free software needs free documentation: a free program should come with manuals
providing the same freedoms that the software does. But this License is not limited to
software manuals; it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License principally for
works whose purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a notice
placed by the copyright holder saying it can be distributed under the terms of this
License. Such a notice grants a world-wide, royalty-free license, unlimited in duration,
to use that work under the conditions stated herein. The "Document", below, refers to
any such manual or work. Any member of the public is a licensee, and is addressed as
"you". You accept the license if you copy, modify or distribute the work in a way
requiring permission under copyright law.
A "Modified Version" of the Document means any work containing the Document or a
portion of it, either copied verbatim, or with modifications and/or translated into
another language.
A "Secondary Section" is a named appendix or a front-matter section of the Document
that deals exclusively with the relationship of the publishers or authors of the Document
to the Document's overall subject (or to related matters) and contains nothing that
could fall directly within that overall subject. (Thus, if the Document is in part a
textbook of mathematics, a Secondary Section may not explain any mathematics.) The
relationship could be a matter of historical connection with the subject or with related
matters, or of legal, commercial, philosophical, ethical or political position regarding
them.
The "Invariant Sections" are certain Secondary Sections whose titles are designated, as
being those of Invariant Sections, in the notice that says that the Document is released
under this License. If a section does not fit the above definition of Secondary then it is
not allowed to be designated as Invariant. The Document may contain zero Invariant
Sections. If the Document does not identify any Invariant Sections then there are none.
The "Cover Texts" are certain short passages of text that are listed, as Front-Cover
Texts or Back-Cover Texts, in the notice that says that the Document is released under
this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may
be at most 25 words.
A "Transparent" copy of the Document means a machine-readable copy, represented in
a format whose specification is available to the general public, that is suitable for
revising the document straightforwardly with generic text editors or (for images
composed of pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or for automatic
translation to a variety of formats suitable for input to text formatters. A copy made in
26 of 32
23/11/06 15:03
an otherwise Transparent file format whose markup, or absence of markup, has been
arranged to thwart or discourage subsequent modification by readers is not
Transparent. An image format is not Transparent if used for any substantial amount of
text. A copy that is not "Transparent" is called "Opaque".
Examples of suitable formats for Transparent copies include plain ASCII without
markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly
available DTD, and standard-conforming simple HTML, PostScript or PDF designed for
human modification. Examples of transparent image formats include PNG, XCF and JPG.
Opaque formats include proprietary formats that can be read and edited only by
proprietary word processors, SGML or XML for which the DTD and/or processing tools
are not generally available, and the machine-generated HTML, PostScript or PDF
produced by some word processors for output purposes only.
The "Title Page" means, for a printed book, the title page itself, plus such following
pages as are needed to hold, legibly, the material this License requires to appear in the
title page. For works in formats which do not have any title page as such, "Title Page"
means the text near the most prominent appearance of the work's title, preceding the
beginning of the body of the text.
A section "Entitled XYZ" means a named subunit of the Document whose title either is
precisely XYZ or contains XYZ in parentheses following text that translates XYZ in
another language. (Here XYZ stands for a specific section name mentioned below, such
as "Acknowledgements", "Dedications", "Endorsements", or "History".) To "Preserve the
Title" of such a section when you modify the Document means that it remains a section
"Entitled XYZ" according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that
this License applies to the Document. These Warranty Disclaimers are considered to be
included by reference in this License, but only as regards disclaiming warranties: any
other implication that these Warranty Disclaimers may have is void and has no effect
on the meaning of this License.
2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or
noncommercially, provided that this License, the copyright notices, and the license
notice saying this License applies to the Document are reproduced in all copies, and
that you add no other conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further copying of the copies
you make or distribute. However, you may accept compensation in exchange for
copies. If you distribute a large enough number of copies you must also follow the
conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may
publicly display copies.
3. COPYING IN QUANTITY
27 of 32
23/11/06 15:03
If you publish printed copies (or copies in media that commonly have printed covers) of
the Document, numbering more than 100, and the Document's license notice requires
Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all
these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the
back cover. Both covers must also clearly and legibly identify you as the publisher of
these copies. The front cover must present the full title with all words of the title equally
prominent and visible. You may add other material on the covers in addition. Copying
with changes limited to the covers, as long as they preserve the title of the Document
and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put
the first ones listed (as many as fit reasonably) on the actual cover, and continue the
rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100,
you must either include a machine-readable Transparent copy along with each Opaque
copy, or state in or with each Opaque copy a computer-network location from which the
general network-using public has access to download using public-standard network
protocols a complete Transparent copy of the Document, free of added material. If you
use the latter option, you must take reasonably prudent steps, when you begin
distribution of Opaque copies in quantity, to ensure that this Transparent copy will
remain thus accessible at the stated location until at least one year after the last time
you distribute an Opaque copy (directly or through your agents or retailers) of that
edition to the public.
It is requested, but not required, that you contact the authors of the Document well
before redistributing any large number of copies, to give them a chance to provide you
with an updated version of the Document.
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions
of sections 2 and 3 above, provided that you release the Modified Version under
precisely this License, with the Modified Version filling the role of the Document, thus
licensing distribution and modification of the Modified Version to whoever possesses a
copy of it. In addition, you must do these things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from that of the
Document, and from those of previous versions (which should, if there were any,
be listed in the History section of the Document). You may use the same title as a
previous version if the original publisher of that version gives permission.
B. List on the Title Page, as authors, one or more persons or entities responsible
for authorship of the modifications in the Modified Version, together with at least
five of the principal authors of the Document (all of its principal authors, if it has
fewer than five), unless they release you from this requirement.
C. State on the Title page the name of the publisher of the Modified Version, as
the publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications adjacent to the other
copyright notices.
28 of 32
23/11/06 15:03
F. Include, immediately after the copyright notices, a license notice giving the
public permission to use the Modified Version under the terms of this License, in
the form shown in the Addendum below.
G. Preserve in that license notice the full lists of Invariant Sections and required
Cover Texts given in the Document's license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled "History", Preserve its Title, and add to it an item
stating at least the title, year, new authors, and publisher of the Modified Version
as given on the Title Page. If there is no section Entitled "History" in the
Document, create one stating the title, year, authors, and publisher of the
Document as given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.
J. Preserve the network location, if any, given in the Document for public access
to a Transparent copy of the Document, and likewise the network locations given
in the Document for previous versions it was based on. These may be placed in
the "History" section. You may omit a network location for a work that was
published at least four years before the Document itself, or if the original publisher
of the version it refers to gives permission.
K. For any section Entitled "Acknowledgements" or "Dedications", Preserve the
Title of the section, and preserve in the section all the substance and tone of each
of the contributor acknowledgements and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered in their text and
in their titles. Section numbers or the equivalent are not considered part of the
section titles.
M. Delete any section Entitled "Endorsements". Such a section may not be
included in the Modified Version.
N. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in
title with any Invariant Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify as
Secondary Sections and contain no material copied from the Document, you may at
your option designate some or all of these sections as invariant. To do this, add their
titles to the list of Invariant Sections in the Modified Version's license notice. These titles
must be distinct from any other section titles.
You may add a section Entitled "Endorsements", provided it contains nothing but
endorsements of your Modified Version by various parties--for example, statements of
peer review or that the text has been approved by an organization as the authoritative
definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up
to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified
Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be
added by (or through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or by arrangement
made by the same entity you are acting on behalf of, you may not add another; but
you may replace the old one, on explicit permission from the previous publisher that
added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission
29 of 32
23/11/06 15:03
to use their names for publicity for or to assert or imply endorsement of any Modified
Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License,
under the terms defined in section 4 above for modified versions, provided that you
include in the combination all of the Invariant Sections of all of the original documents,
unmodified, and list them all as Invariant Sections of your combined work in its license
notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical
Invariant Sections may be replaced with a single copy. If there are multiple Invariant
Sections with the same name but different contents, make the title of each such section
unique by adding at the end of it, in parentheses, the name of the original author or
publisher of that section if known, or else a unique number. Make the same adjustment
to the section titles in the list of Invariant Sections in the license notice of the combined
work.
In the combination, you must combine any sections Entitled "History" in the various
original documents, forming one section Entitled "History"; likewise combine any
sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You
must delete all sections Entitled "Endorsements."
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released
under this License, and replace the individual copies of this License in the various
documents with a single copy that is included in the collection, provided that you follow
the rules of this License for verbatim copying of each of the documents in all other
respects.
You may extract a single document from such a collection, and distribute it individually
under this License, provided you insert a copy of this License into the extracted
document, and follow this License in all other respects regarding verbatim copying of
that document.
7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent
documents or works, in or on a volume of a storage or distribution medium, is called an
"aggregate" if the copyright resulting from the compilation is not used to limit the legal
rights of the compilation's users beyond what the individual works permit. When the
Document is included in an aggregate, this License does not apply to the other works in
the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the
Document, then if the Document is less than one half of the entire aggregate, the
Document's Cover Texts may be placed on covers that bracket the Document within the
30 of 32
23/11/06 15:03
aggregate, or the electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of
the Document under the terms of section 4. Replacing Invariant Sections with
translations requires special permission from their copyright holders, but you may
include translations of some or all Invariant Sections in addition to the original versions
of these Invariant Sections. You may include a translation of this License, and all the
license notices in the Document, and any Warranty Disclaimers, provided that you also
include the original English version of this License and the original versions of those
notices and disclaimers. In case of a disagreement between the translation and the
original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled "Acknowledgements", "Dedications", or
"History", the requirement (section 4) to Preserve its Title (section 1) will typically
require changing the actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly
provided for under this License. Any other attempt to copy, modify, sublicense or
distribute the Document is void, and will automatically terminate your rights under this
License. However, parties who have received copies, or rights, from you under this
License will not have their licenses terminated so long as such parties remain in full
compliance.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free
Documentation License from time to time. Such new versions will be similar in spirit to
the present version, but may differ in detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document
specifies that a particular numbered version of this License "or any later version"
applies to it, you have the option of following the terms and conditions either of that
specified version or of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version number of this
License, you may choose any version ever published (not as a draft) by the Free
Software Foundation.
Retrieved from "http://en.wikibooks.org/wiki/Cell_Biology/Print_version"
Categories: Books with print version | Biology | Cell Biology | Modules that need to be wikified
31 of 32
23/11/06 15:03
This page was last modified 08:47, 17 November 2006.
details).
Privacy policy
About Wikibooks
Disclaimers
32 of 32
23/11/06 15:03
Reading(s)
#1.3
GLOSSARY PQ
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookgloss...
On-Line Biology Book: GLOSSARY
PQ
pacemaker. See sinoatrial node.
Pacinian corpuscles Sensory receptors located deep in the epidermis that detect
pressure and vibration.
paleontology The study of ancient life by collection and analysis of fossils.
Paleozoic Era The period of time beginning 570 million years ago ending 245
million years ago; falls between the Proterozoic and Mesozoic Eras and is divided into
the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.
PICTURE
palindrome A sequence that reads the same in either direction; in genetics, refers to
an enzyme recognition sequence that reads the same on both strands of DNA.
palisade Layer of mesophyll cells in leaves that are closely placed together under the
epidermal layer of the leaf. Palisade parenchyma: Columnar cells located just below
the upper epidermis in leaves the cells where most of the light absorbtion in
photosynthesis occurs. PICTURE 1 | PICTURE 2
palynology The study of palynomorphs and other acid-resistant microfossils usually
produced by plants, protists, and fungi
palynomorph Generic term for any object a palynologist studies.
pancreas A gland in the abdominal cavity that secretes digestive enzymes into the
small intestine and also secretes the hormones insulin and glucagon into the blood,
where they regulate blood glucose levels. A digestive organ that produces trypsin,
chymotrypsin and other enzymes as a pancreatic juice, but which also has endocrine
functions in the production of the hormones somatostatin, insulin, and glucagon.
pancreatic islets Clusters of endocrine cells in the pancreas that secrete insulin and
glucagon; also known as islets of Langerhans.
Pangaea The name proposed by German meteorologist Alfred Wegener for a
supercontinent that existed at the end of the Paleozoic Era and consisted of all the
Earth's landmasses.
parallel evolution The development of similar characteristics in organisms that are
not closely related (not part of a monophyletic group) due to adaptation to similar
environments and/or strategies of life.
parasites Organisms that live in, with, or on another organism. The parasites beneÞt
1 of 13
23/11/06 15:04
GLOSSARY PQ
from the association without contributing to the host, usually they cause some harm to
the host.
parasitism A form of symbiosis in which the population of one species beneÞts at the
expense of the population of another species; similar to predation, but differs in that
parasites act more slowly than predators and do not always kill the host. A type of
symbiosis in which one organism benefits at the expense of the other, for example the
influenza virus is a parasite on its human host. Viruses, are obligate intracellular
parasites.
parasympathetic system The subdivision of the autonomic nervous system that
reverses the effects of the sympathetic nervous system. Part of the autonomic nervous
system that controls heartbeat, respiration and other vital functions.
parenchyma One of the three major cell types in plants. Parenchyma cells have thin,
usually multisided walls, are unspecialized but carry on photosynthesis and cellular
respiration and can store food; form the bulk of the plant body; found in the þeshy
tissue of fruits and seeds, photosynthetic cells of leaves, and the vascular system.
Generalized plant cells whose numerous functions include photosynthesis, storage,
bulk of herbaceous stem tissues, lateral transport in woody stems. Parenchyma are
variously shaped but are characterized by thin walls and remain alive at functional
maturity. PICTURE
parietal lobe The lobe of the cerebral cortex that lies at the top of the brain;
processes information about touch, taste, pressure, pain, and heat and cold. PICTURE
passive transport Diffusion across a plasma membrane in which the cell expends no
energy.
pectin A substance in the middle lamella that cements adjoining plant cells together.
pectoral girdle In humans, the bony arch by which the arms are attached to the rest
of the skeleton; composed of the clavicle and scapula. PICTURE
pedigree analysis A type of genetic analysis in which a trait is traced through several
generations of a family to determine how the trait is inherited. The information is
displayed in a pedigree chart using standard symbols.
pelagic zone One of the two basic subdivisions of the marine biome; consists of the
water above the sea þoor and its organisms.
pelvic girdle In humans, the bony arch by which the legs are attached to the rest of
the skeleton; composed of the two hipbones. PICTURE
pelvis The hollow cavity formed by the two hipbones. PICTURE
penicillin The first of the so-called wonder drugs; discovered by Sir Alexander
Fleming.
2 of 13
23/11/06 15:04
GLOSSARY PQ
pepsin An enzyme produced from pepsinogen that initiates protein digestion by
breaking down protein into large peptide fragments. An enzyme, produced by the
stomach, that chemically breaks down peptide bonds in polypeptides and proteins.
pepsinogen An inactive form of pepsin; synthesized and stored in cells lining the
gastric pits of the stomach.
peptic ulcer Damage to the epithelial layer of the stomach lining; generally caused by
bacterial infection.
peptide bond A covalent bond that links two amino acids together to form a
polypeptide chain. A covalent bond between the amine end of one amino acid and the
acid end of another amino acid. PICTURE
peptides Short chains of amino acids.
perichondrium A layer of connective tissue that forms around the cartilage during
bone formation. Cells in the perichondrium lay down a peripheral layer that develops
into compact bone.
perennials Plants that persist in the environment for more than one year (as in the
case of annuals).
period The fundamental unit in the hierarchy of time units; a part of geologic time
during which a particular sequence of rocks designated as a system was deposited.
Units of geological time that are the major subdivisions of Eras.
periosteum A Þbrous membrane that covers bones and serves as the site of
attachment for skeletal muscles; contains nerves, blood vessels, and lymphatic vessels.
peripheral nervous system The division of the nervous system that connects the
central nervous system to other parts of the body. Components of the nervous system
that transmit messages to the central nervous system.
peristalsis Involuntary contractions of the smooth muscles in the walls of the
esophagus, stomach, and intestines that propel food along the digestive tract. Waves of
muscle contraction in the esophagus that propel food from the oral cavity to the
stomach. PICTURE
Permian Period The last geologic time period of the Paleozoic Era, noted for the
greatest mass extinction in earth history, when nearly 96% of species died out.
PICTURE
peroxisomes Membrane-bound vesicles in eukaryotic cells that contain oxidative
enzymes.
pesticides Chemicals that are applied to agricultural crops or domesticated plants and
which kill or inhibit growth of insects.
3 of 13
23/11/06 15:04
GLOSSARY PQ
petals Usually brightly colored elements of a þower that may produce fragrant oils;
nonreproductive structures that attract pollinators. Sterile leaf-like (white, colorless,
but usually colored) structures in flowers that serve to attract pollinators. PICTURE
petiole The generally non-leafy part of the leaf that attaches the leaf blade to the stem;
celery and rhubarb are examples of a leaf petiole that we use as food. The stalk
connecting the leaf blade to the stem. PICTURE
PGA (phosphoglycerate) A three-carbon molecule formed when carbon dioxide is
added to ribulose biphosphate (RuBP) during the dark reaction of photosynthesis
(Calvin, or Calvin-Benson Cycle). PGA is converted to PGAL, using ATP and
NADPH.
PGAL (phosphoglyceraldehyde) A substance formed from PGA during the dark
reaction of photosynthesis. Some PGAL leaves the cycle and can be converted to
glucose, while other PGAL molecules are used to reform ribulose biphosphate (RuBP)
to continue the dark reaction.
pH The negative logarithm of the H+ ion concentration. The pH is a measure of the
acidity or basic character of a solution. Since it measures a fraction, the larger the pH
number, the less H ions are present in a solution. PICTURE
phagocytes White blood cells that can engulf (by phagocytosis) and destroy
microorganisms including viruses and bacteria; cells in this category include
neutrophils and monocytes.
phagocytosis A form of endocytosis in which white blood cells surround and engulf
invading bacteria or viruses. PICTURE
pharynx The passageway between the mouth and the esophagus and trachea. Food
passes from the pharynx to the esophagus, and air passes from the pharynx to the
trachea.
phenotype The observed properties or outward appearance of a trait. The physical
expression of the alleles posessed by an organism.
pheromones Chemical signals that travel between organisms rather than between cells
within an organism; serve as a form of communication between animals.
phloem Tissue in the vascular system of plants that moves dissolved sugars and other
products of photosynthesis from the leaves to other regions of the plant. Phloem tissue
consists of cells called sieve tubes and companion cells. Cells of the vascular system in
plants that transport food from leaves to other areas of the plant. PICTURE 1 |
PICTURE 2 | PICTURE 3
phosphate group A chemical group composed of a central phosphorous bonded to
three or four oxygens. The net charge on the group is negative. PICTURE
4 of 13
23/11/06 15:04
GLOSSARY PQ
phospholipids Asymmetrical lipid molecules with a hydrophilic head and a
hydrophobic tail. Lipids with a phosphate group in place of one of the three fatty acid
chains. Phospholipids are the building blocks of cellular membranes. Phospholipids
have hydrophilic heads (glycerol and phosphate) and hydrophobic tails (the non-polar
fatty acids). PICTURE
phosphorylation The chemical attachment of phosphorous to a molecule, usually
associated with the storage of energy in the covalent bond that is also formed.
Example: attachment of the third phosphate group to ADP in the formation of the
higher energy form, ATP. Photophosphorylation is a type of phosphorylation
associated with the formation of ATP in the photosynthesis process.
photic zone The layer of the ocean that is penetrated by sunlight; extends to a depth
of about 200 meters.
photoperiodism The ability of certain plants to sense the relative amounts of light
and dark in a 24-hour period; controls the onset of þowering in many plants.
photosynthesis The process by which plant cells use solar energy to produce ATP.
The conversion of unusable sunlight energy into usable chemical energy, associated
with the actions of chlorophyll. PICTURE
photosystems Clusters of several hundred molecules of chlorophyll in a thylakoid in
which photosynthesis takes place. Eukaryotes have two types of photosystems: I and
II. The series of green photoreceptive pigments involved in the light reactions, which
occur in the thylakoids of the chloroplast (in eukaryotes). Energy from light is passed
to the electrons as they move through the photosystem pigments. PICTURE 1 |
PICTURE 2 | PICTURE 3 | PICTURE 4
phototrophs Organisms that use sunlight to synthesize organic nutrients as their
energy source; e.g., cyanobacteria, algae, and plants.
phototropism The reaction of plants to light in which the plants bend toward the
light. Plant response to light by unequal growth caused by concentration of the plant
hormone Indole Acetic Acid (IAA, an auxin) on the darker side of the plant shoot.
PICTURE
phycocyanin An accessory pigment found in cyanobacteria and the chloroplasts of red
algae.
phycoerythrin An accessory pigment found in cyanobacteria and the chloroplasts of
red algae.
phylogeny 1) the study of evolutionary relationships within a monophyletic group. 2)
evolutionary hypotheses represented as a dendrogram or branching diagram.
phylogenetic Pertaining to a phylogeny.
5 of 13
23/11/06 15:04
GLOSSARY PQ
phylum The broadest taxonomic category within kingdoms (pl.: phyla). PICTURE
phytochrome A pigment in plant leaves that detects day length and generates a
response; partly responsible for photoperiodism.
phytoplankton A þoating layer of photosynthetic organisms, including algae, that
are an important source of atmospheric oxygen and form the base of the aquatic food
chain.
pilus Projection from surface of a bacterial cell (F+) that can donate genetic material
to another (F-).
pineal gland A small gland located between the cerebral hemispheres of the brain
that secretes melatonin.
pioneer community The initial community of colonizing species.
pistil Female reproductive structures in flowers, consisting of the stigma, style, and
ovary. Also known as a carpel in some books. PICTURE 1 | PICTURE 2
pith Central area in plant stems, largely composed of parenchyma tissue modified for
storage. PICTURE 1 | PICTURE 2
pituitary gland A small gland located at the base of the brain; consists of an anterior
and a posterior lobe and produces numerous hormones. The master gland of the
endocrine system, the pituitary releases hormones that have specific targets as well as
those that stimulate other glands to secrete hormones. Part of the pituitary is nerve
tissue, the rest is glandular epithelium. PICTURE 1 | PICTURE 2
placenta An organ produced from interlocking maternal and embryonic tissue in
placental mammals; supplies nutrients to the embryo and fetus and removes wastes.
placental mammals One of three groups of mammals that carry their young in the
mother's body for long periods during which the fetus is nourished by the placenta.
Humans are placental mammals.
planaria Small free-living þatworms (Phylum Platyhelminthes) with bilateral
symmetry and cephalization. The freshwater type is often used as an experimental
organism.
planktonic organisms "Floaters"; one of the two main types of organisms in the
pelagic zone of the marine biome.
Plantae The plant kingdom; nonmobile, autotrophic, multicellular eukaryotes.
Kingdom of the plants, autotrophic eukaryotes with cellulose in their cell walls and
starch as a carbohydrate storage product, with chlorophylls a and b as photosynthesis
pigments. PICTURE
plasma The liquid portion of the blood. Along with the extracellular þuid, it makes
6 of 13
23/11/06 15:04
GLOSSARY PQ
up the internal environment of multicellular organisms.
plasma cells Cells produced from B cells that synthesize and release antibodies.
PICTURE
plasmids Self-replicating, circular DNA molecules found in bacterial cells; often
used as vectors in recombinant DNA technology. Small circles of double-stranded
DNA found in some bacteria. Plasmids can carry from four to 20 genes. Plasmids are
a commonly used vector in recombinant DNA studies. PICTURE
plasmodesmata Junctions in plants that penetrate cell walls and plasma membranes,
allowing direct communication between the cytoplasm of adjacent cells (sing.:
plasmodesma).
plasmolysis Osmotic condition in which a cell loses water to its outside environment.
plastids Membrane-bound organelles in plant cells that function in storage (of food
or pigments) or food production. Term for any double membrane-bound organelle.
Chloroplasts contain the chemicals for photosynthesis, amyloplasts (also known as
leukoplasts) store starch, chromoplasts contain colorful pigments such as in the petals
of a flower or epidermis of a fruit.
platelets In vertebrates, cell fragments that bud off from the megakaryocytes in the
bone marrow; carry chemicals needed for blood clotting. Cell fragment functioning in
blood clotting.
plate tectonics The movement of the plates that make up the surface of the Earth.
The revolutionary paradigm in geology that the earth's crust is composed of rigid
segments (plates) in constant (although considered slow in a human-scale time frame)
motion (tectonics) relative to each other.
pleiotropic A term describing a genotype with multiple phenotypic effects. For
example: sickle-cell anemia produces a multitude of consequences in those it affects,
such as heart disease, jidney problem, etc.
Pleistocene The first geologic epoch of the Quaternary Period of the Cenozoic Era
that ended 10,000 years ago with the retreat of the last glaciers. PICTURE
pleura A thin sheet of epithelium that covers the inside of the thoracic cavity and the
outer surface of the lungs.
pleural cavity The space between the sheets of pleura (one covering the inside of the
thoracic cavity, the other covering the outside of the lungs).
polar covalent bond A covalent bond in which atoms share electrons in an unequal
fashion. The resulting molecule has regions with positive and negative charges. The
presence of polar covalent bonds allows other polar molecules to surround molecule:
example: glucose sugar in water.
7 of 13
23/11/06 15:04
GLOSSARY PQ
pollen grains The containers for male gametophytes of seed plants produced in a
microsporangium by meiosis. Microspores produced by seed plants that contain the
male gametophyte. PICTURE
pollen tube Structure produced by the tube nucleus in the pollen grain through which
the sperm nucleus (or nuclei in angiosperms) proceed to travel through to reach the
egg. PICTURE
pollination The transfer of pollen from the anthers to the stigma by a pollinating
agent such as wind, insects, birds, bats, or in a few cases the opening of the flower
itself.
polygenic inheritance Occurs when a trait is controlled by several gene pairs; usually
results in continuous variation. PICTURE
polymer Organic molecule composed of smaller units known as monomers. A large
molecule composed of smaller subunits, for example starch is a polymer of glucose,
proteins are polymers of amino acids.
polymerase chain reaction (PCR) A method of amplifying or copying DNA
fragments that is faster than cloning. The fragments are combined with DNA
polymerase, nucleotides, and other components to form a mixture in which the DNA
is cyclically amplified.
polynucleotides Long chains of nucleotides formed by chemical links between the
sugar and phosphate groups.
polyp The sessile form of life history in cnidarians; e.g., the freshwater hydra.
polyploidy Abnormal variation in the number of chromosome sets. The condition
when a cell or organism has more than the customary two sets of chromosomes. This
is an especially effective speciation mechanism in plants since the extra chromosomes
will establish reproductive isolation with the parental population(s), an essential for
speciation. PICTURE 1 | PICTURE 2
polysaccharides Long chains of monosaccharide units bonded together; e.g.,
glycogen, starch, and cellulose. PICTURE
pons The region that, with the medulla oblongata, makes up the hindbrain, which
controls heart rate, constriction and dilation of blood vessels, respiration, and
digestion. PICTURE
population A group of individuals of the same species living in the same area at the
same time and sharing a common gene pool. A group of potentially interbreeding
organisms in a geographic area.
population dynamics The study of the factors that affect the growth, stability, and
decline of populations, as well as the interactions of those factors.
8 of 13
23/11/06 15:04
GLOSSARY PQ
portal system An arrangement in which capillaries drain into a vein that opens into
another capillary network.
positive feedback Biochemical control where the accumulation of the product
stimulates production of an enzyme responsible for that product's production.
positive feedback control Occurs when information produced by the feedback
increases and accelerates the response.
precambrian Informal term describing 7/8 of geologic time from the beginning of
the earth to the beginning of the Cambrian Period of the Paleozoic Era. During this
time the atmosphere and oceans formed, life originated (or possibly "colonized"
Earth), eukaryotes and simple animals evolved and by the end of the precambrian they
began to accumulate hard preservable parts, the common occurrence of which marks
the beginning of the Cambrian. PICTURE
precipitation The part of the hydrologic cycle in which the water vapor in the
atmosphere falls to Earth as rain or snow.
predation One of the biological interactions that can limit population growth; occurs
when organisms kill and consume other living organisms.
predatory release Occurs when a predator species is removed from a prey species
such as by great reduction in the predator's population size or by the migration of the
prey species to an area without major predators. The removal of the predator releases
the prey from one of the factors limiting its population size.
prehensile movement The ability to seize or grasp.
prenatal testing Testing to detect the presence of a genetic disorder in an embryo or
fetus; commonly done by amniocentesis or chorionic villi sampling.
presymptomatic screening Testing to detect genetic disorders that only become
apparent later in life. The tests are done before the condition actually appears, such as
with Huntington disease.
prey switching The tendency of predators to switch to a more readily available prey
when one prey species becomes rare; allows the Þrst prey population to rebound and
helps prevent its extinction.
primary cell wall The cell wall outside the plasma membrane that surrounds plant
cells; composed of the polysaccharide cellulose.
primary body Those parts of a plant produced by the shoot and root apical
meristems.
primary compounds Chemicals made by plants and needed for the plant's own
metabolism.
9 of 13
23/11/06 15:04
GLOSSARY PQ
primary growth Cells produced by an apical meristem. The growth a plant by the
actions of apical meristems on the shoot and root apices in producing plant primary
tisues.
primary macronutrients Elements that plants require in relatively large quantities:
nitrogen, phosphorus, and potassium.
primary meristems The apical meristems on the shoot and root apices in plants that
produce plant primary tissues.
primary root The Þrst root formed by a plant.
primary structure The sequence of amino acids in a protein. PICTURE
primates The taxonomic order of mammals that includes prosimians (lemurs and
tarsiers), monkeys, apes, and humans; characteristics include large brain, stereoscopic
vision, and grasping hand.
principle of independent assortment Mendel's second law; holds that during gamete
formation, alleles in one gene pair segregate into gametes independently of the alleles
of other gene pairs. As a result, if enough gametes are produced, the collective group
of gametes will contain all combinations of alleles possible for that organism.
principle of segregation Mendel's Þrst law; holds that each pair of factors of heredity
separate during gamete formation so that each gamete receives one member of a pair.
prions Infectious agents composed only of one or more protein molecules without
any accompanying genetic information.
producers The Þrst level in a food pyramid; consist of organisms that generate the
food used by all other organisms in the ecosystem; usually consist of plants making
food by photosynthesis.
progesterone One of the two female reproductive hormones secreted by the ovaries.
prokaryote Type of cell that lacks a membrane-bound nucleus and has no membrane
organelles; a bacterium. Prokaryotes are more primitive than eukaryotes. Cells lacking
membrane-bound organelles and having a single circular chromosome, and ribosomes
surrounded by a cell membrane. Prokaryotes were the first forms of life on earth,
evolving over 3.5 billion years ago.
prolactin A hormone produced by the anterior pituitary; secreted at the end of
pregnancy when it activates milk production by the mammary glands.
promoter The speciÞc nucleotide sequence in DNA that marks the beginning of a
gene. PICTURE
prophase 1) The Þrst stage of mitosis during which chromosomes condense, the
nuclear envelope disappears, and the centrioles divide and migrate to opposite ends of
10 of 13
23/11/06 15:04
GLOSSARY PQ
the cell. 2) The first stage of mitosis and meiosis (although in meiosis this phase is
denoted with either a roman numeral I or II) where the chromatin condenses to form
chromosomes, nucleolus dissolves, nuclear envelope dissolves, and the spindle begins
to form. PICTURE
prostaglandins A class of fatty acids that has many of the properties of hormones;
synthesized and secreted by many body tissues and have a variety of effects on nearby
cells.
prostate gland A gland that is located near and empties into the urethra; produces a
secretion that enhances sperm viability. Gland involved in the reproductive system in
males, the prostate secretes a sperm activating chemical into the semen during the
arousal/ejaculation response. PICTURE
proteinoids Polymers of amino acids formed spontaneously from inorganic
molecules; have enzyme-like properties and can catalyze chemical reactions.
proteins Polymers made up of amino acids that perform a wide variety of cellular
functions. One of the classes of organic macromolecules that function as structural and
control elements in living systems. Proteins are polymers of amino acids linked
together by peptide bonds.
prothallus In ferns, a small heart-shaped bisexual gametophyte. PICTURE
Protista The taxonomic Kingdom from which the other three eukaryotic kingdoms
(Fungi, Animalia and Plantae) are thought to have evolved. The earliest eukaryotes
were single-celled organisms that would today be placed in this admittedly not
monophyletic group. The endosymbiosis theory suggests that eukaryotes may have
evolved independently several times.
protists Single-celled organisms; a type of eukaryote. Protista
proton A subatomic particle in the nucleus of an atom that carries a positive charge.
The positively charged (+1) subatomic particle located in the atomic nucleus and
having a mass slightly less than that of a neutron. Elements differ by the number of
protons in their atoms.
protostomes Animals in which the Þrst opening that appears in the embryo becomes
the mouth; e.g., mollusks, annelids, and arthropods.
protozoa Single-celled protists grouped by their method of locomotion. This group
includes Paramecium, Amoeba, and many other commonly observed protists.
proximal tubule The winding section of the renal tubule where most reabsorption of
water, sodium, amino acids, and sugar takes place. PICTURE
pseudocoelom In nematodes, a closed þuid-containing cavity that acts as a hydrostatic
skeleton to maintain body shape, circulate nutrients, and hold the major body organs.
11 of 13
23/11/06 15:04
GLOSSARY PQ
pseudocoelomates Animals that have a body cavity that is in direct contact with the
outer muscular layer of the body and does not arise by splitting of the mesoderm; e.g.,
roundworms.
pseudopodia Temporary cytoplasmic extensions from a cell that enables it to move
(sing.: pseudopodium). PICTURE
pulmonary artery The artery that carries blood from the right ventricle of the
vertebrate heart to the lungs. Artery carrying oxygen-poor blood from the heart to the
lungs. PICTURE
pulmonary circuit The loop of the circulatory system that carries blood to and from
the lungs. PICTURE
pulmonary vein The vein that carries oxygenated blood from the lungs to the left
atrium of the heart. Veins carrying oxygenated blood from the lungs to the heart.
PICTURE
punctuated equilibrium A model that holds that the evolutionary process is
characterized by long periods with little or no change interspersed with short periods
of rapid speciation.
purine One of the groups of nitrogenous bases that are part of a nucleotide. Purines
are adenine and guanine, and are double-ring structures. PICTURE
pyloric sphincter The ring of muscle at the junction of the stomach and small
intestine that regulates the movement of food into the small intestine. PICTURE
pyrimidine One of the groups of nitrogenous bases that are part of a nucleotide.
Pyrimidines are single ringed, and consist of the bases thymine (in DNA), uracil
(replacing thymine in RNA), and cytosine. PICTURE
quantum models of speciation Models of evolution that hold that speciation
sometimes occurs rapidly as well as over long periods, as the classical theory
proposed.
Quaternary Period The most recent geologic period of the Cenozoic Era, the
Quaternary began 2 million years ago with the growth of northern hemisphere
continental glaciers and the ice age. PICTURE
quaternary structure In some proteins, a fourth structural level created by
interactions with other proteins. Aspect of protein structure determined by the number
and arrangement of polypeptides in a large protein such as hemoglobin. PICTURE
Text ©1992, 1994, 1995, 1997, 1998, 1999, 2000, M.J. Farabee, all rights reserved.
Back to Table of Contents | Back to Main Glossary Page
12 of 13
23/11/06 15:04
GLOSSARY PQ
Email: [email protected]
Last modified:
The URL of this page is:
13 of 13
23/11/06 15:04
Reading(s)
#1.4
CELLS II: CELLULAR ORGANIZATION
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookCELL...
Table of Contents
Cell Size and Shape | The Cell Membrane | The Cell Wall | The Nucleus | Cytoplasm | Vacuoles
and Vesicles | Ribosomes
Endoplasmic Reticulum | Golgi Apparatus and Dictyosomes | Lysosomes | Mitochondria |
Plastids | Cell Movement
Learning Objectives | Terms | Review Questions | Links | References
According to the Cell Theory, all living things are composed of one or more cells.
Cells fall into prokaryotic and eukaryotic types. Prokaryotic cells are smaller (as a
general rule) and lack much of the internal compartmentalization and complexity of
eukaryotic cells. No matter which type of cell we are considering, all cells have certain
features in common: cell membrane, DNA, cytoplasm, and ribosomes.
Cell Size and Shape | Back to Top
The shapes of cells are quite varied with some, such as neurons, being longer than they
are wide and others, such as parenchyma (a common type of plant cell) and
erythrocytes (red blood cells) being equidimensional. Some cells are encased in a rigid
wall, which constrains their shape, while others have a flexible cell membrane (and no
rigid cell wall).
The size of cells is also related to their functions. Eggs (or to use the latin word, ova)
are very large, often being the largest cells an organism produces. The large size of
many eggs is related to the process of development that occurs after the egg is
fertilized, when the contents of the egg (now termed a zygote) are used in a rapid
series of cellular divisions, each requiring tremendous amounts of energy that is
available in the zygote cells. Later in life the energy must be acquired, but at first a
sort of inheritance/trust fund of energy is used.
1 of 25
23/11/06 15:05
2 of 25
23/11/06 15:05
Sizes of viruses, cells, and organisms. Images from Purves et al., Life: The Science of Biology,
4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
The Cell Membrane | Back to Top
The cell membrane functions as a semi-permeable barrier, allowing a very few
molecules across it while fencing the majority of organically produced chemicals
inside the cell. Electron microscopic examinations of cell membranes have led to the
development of the lipid bilayer model (also referred to as the fluid-mosaic model).
The most common molecule in the model is the phospholipid, which has a polar
(hydrophilic) head and two nonpolar (hydrophobic) tails. These phospholipids are
aligned tail to tail so the nonpolar areas form a hydrophobic region between the
hydrophilic heads on the inner and outer surfaces of the membrane. This layering is
termed a bilayer since an electron microscopic technique known as freeze-fracturing is
able to split the bilayer.
Diagram representing the cell membrane. The above image is from
http://www.biosci.uga.edu/almanac/bio_103/notes/may_15.html.
3 of 25
23/11/06 15:05
Cell Membranes from Opposing Neurons (TEM x436,740). This image is copyright Dennis
Kunkel at www.DennisKunkel.com, used with permission.
Cholesterol is another important component of cell membranes embedded in the
hydrophobic areas of the inner (tail-tail) region. Most bacterial cell membranes do not
contain cholesterol.
Proteins are suspended in the inner layer, although the more hydrophilic areas of these
proteins "stick out" into the cells interior and outside of the cell. These proteins
function as gateways that will, in exchange for a price, allow certain molecules to
cross into and out of the cell. These integral proteins are sometimes known as gateway
proteins. The outer surface of the membrane will tend to be rich in glycolipids, which
have their hydrophobic tails embedded in the hydrophobic region of the membrane
and their heads exposed outside the cell. These, along with carbohydrates attached to
the integral proteins, are thought to function in the recognition of self.
The contents (both chemical and organelles)of the cell are termed protoplasm, and are
further subdivided into cytoplasm (all of the protoplasm except the contents of the
nucleus) and nucleoplasm (all of the material, plasma and DNA etc. within the
nucleus).
The Cell Wall | Back to Top
Not all living things have cell walls, most notably animals and many of the more
animal-like Protistans. Bacteria have cell walls containing peptidoglycan. Plant cells
4 of 25
23/11/06 15:05
have a variety of chemicals incorporated in their cell walls. Cellulose is the most
common chemical in the plant primary cell wall. Some plant cells also have lignin and
other chemicals embedded in their secondary walls. The cell wall is located outside the
plasma membrane. Plasmodesmata are connections through which cells communicate
chemically with each other through their thick walls. Fungi and many protists have
cell walls although they do not contain cellulose, rather a variety of chemicals (chitin
for fungi).
Structure of a typical plant cell. Image from Purves et al., Life: The Science of Biology, 4th
Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used
with permission.
5 of 25
23/11/06 15:05
Lily Parenchyma Cell (cross-section) (TEM x7,210). Note the large nucleus and nucleolus in
the center of the cell, mitochondria and plastids in the cytoplasm. This image is copyright Dennis
Structure of an animal cell. The above image is from
http://www.biosci.uga.edu/almanac/bio_103/notes/may_15.html.
6 of 25
23/11/06 15:05
Liver Cell (TEM x9,400). This image is copyright Dennis Kunkel This image is copyright Dennis
The nucleus | Back to Top
The nucleus occurs only in eukaryotic cells, and is the location of the majority of
different types of nucleic acids. Van Hammerling's experiment (click here for a
diagram) showed the role of the nucleus in controlling the shape and features of the
cell. Deoxyribonucleic acid, DNA, is the physical carrier of inheritance and with the
exception of plastid DNA (cpDNA and mDNA, see below) all DNA is restricted to the
nucleus. Ribonucleic acid, RNA, is formed in the nucleus by coding off of the DNA
bases. RNA moves out into the cytoplasm. The nucleolus is an area of the nucleus
(usually 2 nucleoli per nucleus) where ribosomes are constructed.
7 of 25
23/11/06 15:05
Structure of the nucleus. Note the chromatin, uncoiled DNA that occupies the space
within the nuclear envelope. Image from Purves et al., Life: The Science of Biology, 4th Edition,
by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with
permission.
Liver cell nucleus and nucleolus (TEM x20,740). Cytoplasm, mitochondria, endoplasmic
reticulum, and ribosomes also shown.This image is copyright Dennis Kunkel at
www.DennisKunkel.com, used with permission.
8 of 25
23/11/06 15:05
The nuclear envelope is a double-membrane structure. Numerous pores occur in the
envelope, allowing RNA and other chemicals to pass, but the DNA not to pass.
Structure of the nuclear envelope and nuclear pores. Image from Purves et al., Life: The
Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman
(www.whfreeman.com), used with permission.
Nucleus with Nuclear Pores (TEM x73,200). The cytoplasm also contains numerous
ribosomes. This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
9 of 25
23/11/06 15:05
Cytoplasm | Back to Top
The cytoplasm was defined earlier as the material between the plasma membrane (cell
membrane) and the nuclear envelope. Fibrous proteins that occur in the cytoplasm,
referred to as the cytoskeleton maintain the shape of the cell as well as anchoring
organelles, moving the cell and controlling internal movement of structures.
Microtubules function in cell division and serve as a "temporary scaffolding" for other
organelles. Actin filaments are thin threads that function in cell division and cell
motility. Intermediate filaments are between the size of the microtubules and the actin
filaments.
Actin and tubulin components of the cytoskeleton. Image from Purves et al., Life: The Science
of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman
Vacuoles and vesicles | Back to Top
Vacuoles are single-membrane organelles that are essentially part of the outside that is
located within the cell. The single membrane is known in plant cells as a tonoplast.
Many organisms will use vacuoles as storage areas. Vesicles; are much smaller than
vacuoles and function in transport within and to the outside of the cell.
Ribosomes | Back to Top
Ribosomes are the sites of protein synthesis. They are not membrane-bound and thus
occur in both prokaryotes and eukaryotes. Eukaryotic ribosomes are slightly larger
than prokaryotic ones. Structurally the ribosome consists of a small and larger subunit.
10 of 25
23/11/06 15:05
Biochemically the ribosome consists of ribosomal RNA (rRNA) and some 50
structural proteins. Often ribosomes cluster on the endoplasmic reticulum, in which
case they resemble a series of factories adjoining a railroad line. Click here for
Ribosomes (More than you ever wanted to know about ribosomes!)
Structure of the ribosome. Image from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with
permission.
11 of 25
23/11/06 15:05
Ribosomes and Polyribosomes - liver cell (TEM x173,400). This image is copyright Dennis
Endoplasmic reticulum | Back to Top
Endoplasmic reticulum is a mesh of interconnected membranes that serve a function
involving protein synthesis and transport. Rough endoplasmic reticulum (Rough ER)
is so-named because of its rough appearance due to the numerous ribosomes that occur
along the ER. Rough ER connects to the nuclear envelope through which the
messenger RNA (mRNA) that is the blueprint for proteins travels to the ribosomes.
Smooth ER; lacks the ribosomes characteristic of Rough ER and is thought to be
involved in transport and a variety of other functions.
The endoplasmic reticulum. Rough endoplasmic reticulum is on the left, smooth
endoplasmic reticulum is on the right. Image from Purves et al., Life: The Science of Biology,
12 of 25
23/11/06 15:05
Rough Endoplasmic Reticulum with Ribosomes (TEM x61,560). This image is copyright
Dennis Kunkel at www.DennisKunkel.com, used with permission.
Golgi Apparatus and Dictyosomes | Back to Top
Golgi Complexes are flattened stacks of membrane-bound sacs. They function as a
packaging plant, modifying vesicles from the Rough ER. New membrane material is
assembled in various cisternae of the golgi.
13 of 25
23/11/06 15:05
Structure of the Golgi apparatus and its functioning in vesicle-mediated transport.
Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
14 of 25
23/11/06 15:05
Golgi Apparatus in a plant parenchyma cell from Sauromatum guttatum (TEM
x145,700). Note the numerous vesicles near the Golgi. This image is copyright Dennis
Lysosomes | Back to Top
Lysosomes are relatively large vesicles formed by the Golgi. They contain hydrolytic
enzymes that could destroy the cell. Lysosome contents function in the extracellular
breakdown of materials.
15 of 25
23/11/06 15:05
Role of the Golgi in forming lysosomes. Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman
Mitochondria | Back to Top
Mitochondria contain their own DNA (termed mDNA) and are thought to represent
bacteria-like organisms incorporated into eukaryotic cells over 700 million years ago
(perhaps even as far back as 1.5 billion years ago). They function as the sites of
energy release (following glycolysis in the cytoplasm) and ATP formation (by
chemiosmosis). The mitochondrion has been termed the powerhouse of the cell.
Mitochondria are bounded by two membranes. The inner membrane folds into a series
of cristae, which are the surfaces on which ATP is generated.
Structure of a mitochondrion. Image from Purves et al., Life: The Science of Biology, 4th Edition,
16 of 25
23/11/06 15:05
permission.
Muscle Cell Mitochondrion (TEM x190,920). This image is copyright Dennis Kunkel at
Mitochondria and endosymbiosis
During the 1980s, Lynn Margulis proposed the theory of endosymbiosis to explain the
origin of mitochondria and chloroplasts from permanent resident prokaryotes.
According to this idea, a larger prokaryote (or perhaps early eukaryote) engulfed or
surrounded a smaller prokaryote some 1.5 billion to 700 million years ago.
The basic events in endosymbiosis. Image from Purves et al., Life: The Science of Biology, 4th
17 of 25
23/11/06 15:05
with permission.
Instead of digesting the smaller organisms the large one and the smaller one entered
into a type of symbiosis known as mutualism, wherein both organisms benefit and
neither is harmed. The larger organism gained excess ATP provided by the
"protomitochondrion" and excess sugar provided by the "protochloroplast", while
providing a stable environment and the raw materials the endosymbionts required.
This is so strong that now eukaryotic cells cannot survive without mitochondria
(likewise photosynthetic eukaryotes cannot survive without chloroplasts), and the
endosymbionts can not survive outside their hosts. Nearly all eukaryotes have
mitochondria. Mitochondrial division is remarkably similar to the prokaryotic methods
that will be studied later in this course. A summary of the theory is available by
clicking here.
Plastids | Back to Top
Plastids are also membrane-bound organelles that only occur in plants and
photosynthetic eukaryotes.
Chloroplasts are the sites of photosynthesis in eukaryotes. They contain chlorophyll,
the green pigment necessary for photosynthesis to occur, and associated accessory
pigments (carotenes and xanthophylls) in photosystems embedded in membranous
sacs, thylakoids (collectively a stack of thylakoids are a granum [plural = grana])
floating in a fluid termed the stroma. Chloroplasts contain many different types of
accessory pigments, depending on the taxonomic group of the organism being
observed.
18 of 25
23/11/06 15:05
Structure of the chloroplast. Image from Purves et al., Life: The Science of Biology, 4th Edition,
permission.
Chloroplasts and endosymbiosis
Like mitochondria, chloroplasts have their own DNA, termed cpDNA. Chloroplasts of
Green Algae (Protista) and Plants (descendants of some Green Algae) are thought to
have originated by endosymbiosis of a prokaryotic alga similar to living Prochloron
(Prochlorobacteria). Chloroplasts of Red Algae (Protista) are very similar
biochemically to cyanobacteria (also known as blue-green bacteria [algae to
chronologically enhanced folks like myself :)]). Endosymbiosis is also invoked for this
similarity, perhaps indicating more than one endosymbiotic event occurred.
Leukoplasts store starch, sometimes protein or oils.
Chromoplasts store pigments associated with the bright colors of flowers and/or fruits.
Cell Movement | Back to Top
Cell movement; is both internal, referred to as cytoplasmic streaming and external,
referred to as motility. Internal movements of organelles are governed by actin
filaments. These filaments make an area in which organelles such as chloroplasts can
move. Internal movement is known as cytoplasmic streaming. External movement of
19 of 25
23/11/06 15:05
cells is determined by special organelles for locomotion.
The cytoskeleton. Image from Prentice Hall.
Cilia and flagella are similar except for length, cilia being much shorter. They both
have the characteristic 9 + 2 arrangement of microtubules.
The 9+2 arrangement of microtubules in a flagellum or cilium. Image from Prentice Hall.
20 of 25
23/11/06 15:05
Cilia from an epithelial cell in cross section (TEM x199,500). Note the 9 + 2
arrangement of cilia. This image is copyright Dennis Kunkel at www.DennisKunkel.com, used
with permission.
Flagella work as whips pulling (as in Chlamydomonas or Halosphaera) or pushing
(dinoflagellates, a group of single-celled Protista) the organism through the water.
Cilia work like oars on a viking longship (Paramecium has 17,000 such oars covering
its outer surface).
21 of 25
23/11/06 15:05
Movement of cilia and flagella. Image from Purves et al., Life: The Science of Biology, 4th
with permission.
Pseudopodia are used by many cells, such as Amoeba, Chaos (Pelomyxa) and human
leukocytes (white blood cells). These are not structures as such but rather are
associated with actin near the moving edge.
Formation and functioning of a pseudopod by an amoeboid cell. Image from Purves et al.,
Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman
Learning Objectives | Back to Top
22 of 25
23/11/06 15:05
Give the function and cellular location of the following basic eukaryotic organelles and structures:
cell membrane, nucleus, endoplasmic reticulum, Golgi bodies, lysosomes, mitochondria,
ribosomes, chloroplasts, vacuoles, and cell walls.
A micrometer is one-millionth of a meter long. A nanometer is one-billionth of a meter long. How
many micrometers tall are you?
Describe the function of the nuclear envelope and nucleolus.
Describe the details of the structure of the chloroplast, the site of photosynthesis.
Mature, living plant cells often have a large, fluid-filled central vacuole that can store amino acids,
sugars, ions, and toxic wastes. Animal cells generally lack large vacuoles. How do animal cells
perform these functions?
Microtubules, microfilaments, and intermediate filaments are all main components of the
cytoskeleton.
Flagella and cilia propel eukaryotic cells through their environment; the microtubule organization
in these organelles is a 9+2 array.
Terms | Back to Top
actin
carotenes
chlorophyll cristae
endoplasmic erythrocytes
reticulum
Green Algae hydrophilic
mitochondria mutualism
parenchyma phospholipid
Red Algae ribosomal
RNA
vacuoles
zygote
cellulose
cell walls
cyanobacteria cytoplasm
chemiosmosis chitin
cytoskeleton dinoflagellates
eukaryotic fluid-mosaic Golgi
grana
complexes
hydrophobic leukocytes
lysosomes
microtubules
neurons
nucleus
nucleolus
ova
photosystems plasmodesmata plastid
pseudopodia
ribosomes
stroma
symbiosis
thylakoids
Review Questions | Back to Top
1. There are ____ micrometers (µm) in one millimeter (mm). a) 1; b) 10; c) 100; d) 1000; e) 1/1000
2. Human cells have a size range between ___ and ___ micrometers (µm). a) 10-100; b) 1-10; c)
100-1000; d) 1/10-1/1000
3. Chloroplasts and bacteria are ___ in size. a) similar; b) at different ends of the size range; c)
exactly the same; d) none of these.
4. The plasma membrane does all of these except ______. a) contains the hereditary material; b) acts
as a boundary or border for the cytoplasm; c) regulates passage of material in and out of the cell;
d) functions in the recognition of self
5. Which of these materials is not a major component of the plasma membrane? a) phospholipids; b)
glycoproteins; c) proteins; d) DNA
6. Cells walls are found in members of these kingdoms, except for ___, which all lack cell walls. a)
plants; b) animals; c) bacteria; d) fungi
7. The polysaccharide ___ is a major component of plan cell walls. a) chitin; b) peptidoglycan; c)
cellulose; d) mannitol; e) cholesterol
8. Plant cells have ___ and ___, which are not present in animal cells. a) mitochondria, chloroplasts;
b) cell membranes, cell walls; c) chloroplasts, nucleus; d) chloroplasts, cell wall
9. The ___ is the membrane enclosed structure in eukaryotic cells that contains the DNA of the cell.
a) mitochondrion; b) chloroplast; c) nucleolus; d) nucleus
10. Ribosomes are constructed in the ___. a) endoplasmic reticulum; b) nucleoid; c) nucleolus; d)
23 of 25
23/11/06 15:05
nuclear pore
11. Rough endoplasmic reticulum is the area in a cell where ___ are synthesized. a) polysaccharides;
b) proteins; c) lipids; d) DNA
12. The smooth endoplasmic reticulum is the area in a cell where ___ are synthesized. a)
polysaccharides; b) proteins; c) lipids; d) DNA
13. The mitochondrion functions in ____. a) lipid storage; b) protein synthesis; c) photosynthesis; d)
DNA replication; e) ATP synthesis
14. The thin extensions of the inner mitochondrial membrane are known as _____. a) cristae; b)
matrix; c) thylakoids; d) stroma
15. The chloroplast functions in ____. a) lipid storage; b) protein synthesis; c) photosynthesis; d)
DNA replication; e) ATP synthesis
16. Which of these cellular organelles have their own DNA? a) chloroplast; b) nucleus; c)
mitochondrion; d) all of these
17. The theory of ___ was proposed to explain the possible origin of chloroplasts and mitochondria.
a) evolution; b) endosymbiosis; c) endocytosis; d) cells
18. Long, whiplike microfibrils that facilitate movement by cells are known as ___. a) cilia; b)
flagella; c) leather; d) pseudopodia
Links | Back to Top
Protist Image Data: Pictures and resources about Protista.
Cell Biology Lab Manual: Lab protocols and links pertaining to cell biology. A nice place to look
for new things to do in labs.
MIT Hypertextbook Chapter on Cell Biology: Excellent site with illustrations and additional
details to complement the above material.
Virtual Plant Cell: Zoom in on a virtual plant cell. An excellent first step.
WWW Cell Biology Course: An excellent site employing image maps and details about cell
biology.
Dictionary of Cell Biology: A searchable dictionary pertinent to this topic.
Cells Alive! Very interesting site with new features each month.
Ribosomes A text with links to illustrations. More than you ever wanted to know about
ribosomes!
Charlie and Bobby Jo's Excellent Adventure Look out, honey, I shrunk the lab! Humorous
journey through a cell. (Dr. Sata, University of Texas).
The Cell Nucleus This easily navigated series of well illustrated pages presents an impressive
amount of information about the various functions carried out by the nucleus.
References | Back to Top
Text ©1992, 1994, 1997, 1998, 1999, 2000, 2001, by M.J. Farabee, all rights reserved. Use for
educational purposes is heartily encouraged.
Back to Table of Contents | Go to TRANSPORT IN AND OUT OF CELLS
Last modified:
24 of 25
23/11/06 15:05
25 of 25
23/11/06 15:05
Reading(s) #2
Mitochondrion - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Mitochondria
Mitochondrion
From Wikipedia, the free encyclopedia
(Redirected from Mitochondria)
In cell biology, a mitochondrion (plural mitochondria) (from Greek μιτος
or mitos, thread + κουδριον or khondrion, granule) is a membrane-enclosed
organelle, found in most eukaryotic cells.[1] Mitochondria are sometimes
described as "cellular power plants," because they convert food molecules into
energy in the form of ATP via the process of oxidative phosphorylation. A
typical eukaryotic cell contains about 2,000 mitochondria, which occupy
roughly one fifth of its total volume.[2] Mitochondria contain DNA that is
independent of the DNA located in the cell nucleus. According to the
endosymbiotic theory, mitochondria are descended from free-living
prokaryotes.
Electron micrograph of a
mitochondrion showing its
mitochondrial matrix and
membranes
Contents
1 Mitochondrion structure
1.1 Outer membrane
1.2 Inner membrane
1.3 Mitochondrial matrix
2 Mitochondrial functions
2.1 Energy conversion
2.1.1 Pyruvate: the citric acid cycle
2.1.2 NADH and FADH2: the electron transport chain
2.1.3 Heat production
2.1.4 Storage for calcium ions
3 Origin
4 Replication and gene inheritance
5 Use in population genetic studies
6 Fiction
7 Cited references
8 General references
9 External links
10 See also
Mitochondrion structure
1 of 8
23/11/06 15:06
A mitochondrion contains inner and outer
membranes composed of phospholipid
bilayers and proteins. The two membranes,
however, have different properties. Because
of this double-membraned organization,
there are 5 distinct compartments within
mitochondria. There is the outer membrane,
the intermembrane space (the space between
the outer and inner membranes), the inner
membrane, the cristae space (formed by
infoldings of the inner membrane), and the
matrix (space within the inner membrane).
Mitochondria range from 1 to 10
micrometers (μm) in size.
Outer membrane
Simplified structure of a typical mitochondrion
The outer mitochondrial membrane, which
encloses the entire organelle, has a
protein-to-phospholipid ratio similar to the eukaryotic plasma membrane (about 1:1 by weight). It contains
numerous integral proteins called porins, which contain a relatively large internal channel (about 2-3 nm)
that is permeable to all molecules of 5000 daltons or less.[3] Larger molecules can only traverse the outer
membrane by active transport. It also contains enzymes involved in such diverse activities as the elongation
of fatty acids, oxidation of epinephrine (adrenaline), and the degradation of tryptophan.
Inner membrane
The inner mitochondrial membrane contains proteins with four types of functions: [3]
1.
2.
3.
4.
Those that carry out the oxidation reactions of the respiratory chain.
ATP synthase, which makes ATP in the matrix.
Specific transport proteins that regulate the passage of metabolites into and out of the matrix.
Protein import machinery.
It contains more than 100 different polypeptides, and has a very high protein-to-phospholipid ratio (more
than 3:1 by weight, which is about 1 protein for 15 phospholipids). Additionally, the inner membrane is rich
in an unusual phospholipid, cardiolipin, which is usually characteristic of bacterial plasma membranes.
Unlike the outer membrane, the inner membrane does not contain porins, and is highly impermeable; almost
all ions and molecules require special membrane transporters to enter or exit the matrix. In addition, there is
a membrane potential across the inner membrane.
The inner mitochondrial membrane is compartmentalized into numerous cristae, which expand the surface
area of the inner mitochondrial membrane, enhancing its ability to generate ATP. In typical liver
mitochondria, for example, the surface area, including cristae, is about five times that of the outer
membrane. Mitochondria of cells which have greater demand for ATP, such as muscle cells, contain more
cristae than typical liver mitochondria.
Mitochondrial matrix
2 of 8
23/11/06 15:06
The matrix is the space enclosed by the inner membrane. The
matrix contains a highly concentrated mixture of hundreds of
enzymes, in addition to the special mitochondrial ribosomes, tRNA,
and several copies of the mitochondrial DNA genome. Of the
enzymes, the major functions include oxidation of pyruvate and
fatty acids, and the citric acid cycle.[3]
Image of cristae in rat liver
mitochondrion
Mitochondria possess their own genetic material, and the machinery
to manufacture their own RNAs and proteins. (See: protein
synthesis). This nonchromosomal DNA encodes a small number of
mitochondrial peptides (13 in humans) that are integrated into the
inner mitochondrial membrane, along with proteins encoded by
genes that reside in the host cell's nucleus.
Mitochondrial functions
Although it is well known that the mitochondria convert organic materials into cellular energy in the form of
ATP, mitochondria play an important role in many metabolic tasks, such as:
Apoptosis-programmed cell death
Glutamate-mediated excitotoxic neuronal injury
Cellular proliferation
Regulation of the cellular redox state
Heme synthesis
Steroid synthesis
Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in
liver cells contain enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. A
mutation in the genes regulating any of these functions can result in mitochondrial diseases.
Energy conversion
A dominant role for the mitochondria is the production of ATP as reflected by the large number of proteins
in the inner membrane for this task. This is done oxidising the major products of glycolysis: pyruvate and
NADH that are produced in the cytosol. This process of cellular respiration, also known as aerobic
respiration, is dependent on the presence of oxygen. When oxygen is limited the glycolytic products will be
metabolised by anaerobic respiration a process that is independent of the mitochondria. The production of
ATP from glucose has an approximately 15 fold higher yield during aerobic respiration compared to
anaerobic respiration.
Pyruvate: the citric acid cycle
Each pyruvate molecule produced by glycolysis is actively transported across the inner mitochondrial
membrane, and into the matrix where it is oxidized and combined with coenzyme A to form CO2 , acetyl
CoA and NADH.
The acetyl CoA is the primary substrate to enter the citric acid cycle , also known as the tricarboxylic acid
(TCA) cycle or Krebs cycle. The enzymes of the citric acid cycle are located in the mitochondrial matrix with
the exception of succinate dehydrogenase, which is bound to the inner mitochondrial membrane. The citric
acid cycle oxidises the acetyl CoA to carbon dioxide and in the process produces reduced cofactors (three
molecules of NADH and one molecule of FADH2 ), that are a source of electrons for the electron transport
3 of 8
23/11/06 15:06
chain, and a molecule of GTP (that is readily converted to an ATP).
NADH and FADH2 : the electron transport chain
Pyruvate (from glycolysis) is made within the animal cell
within the fluid.
The redox energy from NADH and FADH2 is transferred to
oxygen (O2) in several steps via the electron transport chain.
These energy-rich molecules are produced within the matrix
via the citric acid cycle but are also produced in the
cytoplasm by glycolysis; reducing equivalents from the
cytoplasm can be imported via the malate-aspartate shuttle
system of antiporter proteins or feed into the electron
transport chain using a glycerol phosphate shuttle. Protein
Schematic of typical animal cell, showing
complexes in the inner membrane (NADH dehydrogenase,
subcellular components. Organelles: (1) nucleolus
cytochrome c reductase and cytochrome c oxidase) perform
(2) nucleus (3) ribosome (4) vesicle (5) rough
the transfer and the incremental release of energy is used to
endoplasmic reticulum (ER) (6) Golgi apparatus (7)
Cytoskeleton (8) smooth ER (9) mitochondria
pump protons (H+) into the intermembrane space. This
(10) vacuole (11) cytoplasm (12) lysosome (13)
process is efficient but a small percentage of electrons may
centrioles
prematurely reduce oxygen, forming the toxic free radical
superoxide. This can cause oxidative damage in the
mitochondria and may contribute to the decline in mitochondrial function associated with the aging
process.[4]
As the proton concentration increases in the intermembrane space, a strong electrochemical gradient is
established across the inner membrane. The protons can return to the matrix through the ATP synthase
complex and their potential energy is used to synthesize ATP from ADP and inorganic phosphate (Pi ). This
process is called chemiosmosis and was first described by Peter Mitchell who was awarded the 1978 Nobel
Prize in Chemistry for his work. Later, part of the 1997 Nobel Prize in Chemistry was awarded to Paul D.
Boyer and John E. Walker for their clarification of the working mechanism of ATP synthase.
Heat production
Under certain conditions, protons can re-enter the mitochondrial matrix without contributing to ATP
synthesis. This process is known as proton leak or mitochondrial uncoupling and is due to the facilitated
diffusion of protons into the matrix, mediated by a proton channel called thermogenin. This results in the
unharnessed potential energy of the proton electrochemical gradient being released as heat. Thermogenin is
found in brown adipose tissue (brown in colour due to high levels of mitochondria) where it is used to
generate heat by non-shivering thermogenesis. Non-shivering thermogenesis is the primary means of heat
generation in newborn or hibernating mammals.
Storage for calcium ions
The concentrations of free calcium in the cell can regulate an array of reactions and is important for signal
transduction in the cell. Mitochondria store free calcium, a process that is one important event for the
homestasis of calcium in the cell. Release of this calcium back into the cells interior can initiate calcium
spikes or waves. These events coordinate processes such as neurotransmitter release in nerve cells and release
of hormones in endocrine cells.
4 of 8
23/11/06 15:06
Origin
As mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, it
is generally accepted that they were originally derived from endosymbiotic prokaryotes. Studies of
mitochondrial DNA, which is often circular and employs a variant genetic code, show their ancestor, the
so-called proto-mitochondrion, was a member of the Proteobacteria.[5] In particular, the pre-mitochondrion
was probably related to the rickettsias, although the exact position of the ancestor of mitochondria among
the alpha-proteobacteria remains controversial. The endosymbiotic hypothesis suggests that mitochondria
descended from specialized bacteria (probably purple non-sulfur bacteria) that somehow survived
endocytosis by another species of prokaryote or some other cell type, and became incorporated into the
cytoplasm. The ability of symbiont bacteria to conduct cellular respiration in host cells that had relied on
glycolysis and fermentation would have provided a considerable evolutionary advantage. Similarly, host
cells with symbiotic bacteria capable of photosynthesis would also have an advantage. In both cases, the
number of environments in which the cells could survive would have been greatly expanded.
This relationship developed at least 2 billion years ago and mitochondria still show some signs of their
ancient origin. Mitochondrial ribosomes are the 70S (bacterial) type, in contrast to the 80S ribosomes found
elsewhere in the cell. As in prokaryotes, there is a very high proportion of coding DNA, and an absence of
repeats. Mitochondrial genes are transcribed as multigenic transcripts which are cleaved and polyadenylated
to yield mature mRNAs. Unlike their nuclear cousins, mitochondrial genes are small, generally lacking
introns, and many chromosomes are circular, conforming to the bacterial pattern.
A few groups of unicellular eukaryotes lack mitochondria: the microsporidians, metamonads, and
archamoebae. On rRNA trees these groups appeared as the most primitive eukaryotes, suggesting they
appeared before the origin of mitochondria, but this is now known to be an artifact of long branch attraction
— they are apparently derived groups and retain genes or organelles derived from mitochondria (e.g.
mitosomes and hydrogenosomes).[1] There are no primitively amitochondriate eukaryotes, and so the origin
of mitochondria may have played a critical part in the development of eukaryotic cells.
Replication and gene inheritance
Mitochondria replicate their DNA and divide mainly in response to the energy needs of the cell; in other
words, their growth and division is not linked to the cell cycle. When the energy needs of a cell are high,
mitochondria grow and divide. When the energy use is low, mitochondria are destroyed or become inactive.
At cell division, mitochondria are distributed to the daughter cells more or less randomly during the division
of the cytoplasm. Mitochondria divide by binary fission similar to bacterial cell division. Unlike bacteria,
however, mitochondria can also fuse with other mitochondria. Sometimes new mitochondria are synthesized
in centers that are rich in the proteins and polysomes needed for their synthesis.
Mitochondrial genes are not inherited by the same mechanism as nuclear genes. At fertilization of an egg by
a sperm, the egg nucleus and sperm nucleus each contribute equally to the genetic makeup of the zygote
nucleus. In contrast, the mitochondria, and therefore the mitochondrial DNA, usually comes from the egg
only. The sperm's mitochondria enters the egg, but are almost always destroyed and do not contribute their
genes to the embryo.[6] Paternal sperm mitochondria are marked with ubiquitin to select them for later
destruction inside the embryo.[7] The egg contains relatively few mitochondria, but it is these mitochondria
that survive and divide to populate the cells of the adult organism. Mitochondria are, therefore, in most cases
inherited down the female line.
This maternal inheritance of mitochondrial DNA is seen in most organisms, including all animals. However,
5 of 8
23/11/06 15:06
mitochondria in some species can sometimes be inherited through the father. This is the norm amongst
certain coniferous plants (although not in pines and yew trees).[8] It has been suggested to occur at a very
low level in humans.[9]
Uniparental inheritance means that there is little opportunity for genetic recombination between different
lineages of mitochondria. For this reason, mitochondrial DNA is usually thought of as reproducing by
binary fission. However, there are several claims of recombination in mitochondrial DNA, most
controversially in humans. If recombination does not occur, the whole mitochondrial DNA sequence
represents a single haplotype, which makes it useful for studying the evolutionary history of populations.
Mitochondrial genomes have many fewer genes than do the related eubacteria from which they are thought
to be descended. Although some have been lost altogether, many seem to have been transferred to the
nucleus. This is thought to be relatively common over evolutionary time. A few organisms, such as
Cryptosporidium, actually have mitochondria which lack any DNA, presumably because all their genes have
either been lost or transferred.
The uniparental inheritance of mitochondria is thought to result in intragenomic conflict, such as seen in the
petite mutant mitochondria of some yeast species. It is possible that the evolution of separate male and
female sexes is a mechanism to resolve this organelle conflict.
Use in population genetic studies
The near-absence of genetic recombination in mitochondrial DNA makes it a useful source of information
for scientists involved in population genetics and evolutionary biology. Because all the mitochondrial DNA
is inherited as a single unit, or haplotype, the relationships between mitochondrial DNA from different
individuals can be represented as a gene tree. Patterns in these gene trees can be used to infer the
evolutionary history of populations. The classic example of this is in human evolutionary genetics, where the
molecular clock can be used to provide a recent date for mitochondrial Eve. This is often interpreted as
strong support for a recent modern human expansion out of Africa. Another human example is the
sequencing of mitochondrial DNA from Neanderthal bones. The relatively large evolutionary distance
between the mitochondrial DNA sequences of Neanderthals and living humans has been interpreted as
evidence for lack of interbreeding between Neanderthals and anatomically modern humans.
However, mitochondrial DNA only reflects the history of females in a population, and so may not give a
representative picture of the history of the population as a whole. For example, if dispersal is primarily
undertaken by males, this will not be picked up by mitochondrial studies. This can be partially overcome by
the use of patrilineal genetic sequences, if they are available (in mammals the non-recombining region of the
Y-chromosome provides such a source). More broadly, only studies that also include nuclear DNA can
provide a comprehensive evolutionary history of a population; unfortunately, genetic recombination means
that these studies can be difficult to analyse.
Fiction
The midi-clorians of the Star Wars universe are fictional life-forms inside cells that provide the Force.
George Lucas took inspiration from the endosymbiotic theory.
Madeleine L'Engle's novel A Wind in the Door posits fictional "farandolae" which are to mitochondria
what mitochondria are to cells.
In Hideaki Sena's novel Parasite Eve (and the video game based on it), mitochondria are independent
organisms, using animals and plants as a form of "transportation," causing a major biological disaster
when they decide to set themselves free.
6 of 8
23/11/06 15:06
Cited references
1. ^ a b Henze, K., W. Martin (2003). "Evolutionary biology: Essence of mitochondria". Nature 426:
127-128.
2. ^ Voet, Donald, Judith G. Voet, Charlotte W. Pratt (2006). Fundamentals of Biochemistry, 2nd Edition.
John Wiley and Sons, Inc., 547. ISBN 0-471-21495-7.
3. ^ a b c Alberts, Bruce, et. al. (1994). Molecular Biology of the Cell. New York: Garland Publishing
Inc..
4. ^ Huang, K., K. G. Manton (2004). "The role of oxidative damage in mitochondria during aging: A
review". Frontiers in Bioscience 9: 1100-1117.
5. ^ Futuyma, Douglas J. (2005). "On Darwin's Shoulders". Natural History 114 (9): 64–68.
6. ^ Kimball, J.W. (2006) "Sexual Reproduction in Humans: Copulation and Fertilization,"
(http://home.comcast.net/~john.kimball1/BiologyPages/S/Sexual_Reproduction.html#Copulation_and_Fertilization
Kimball's Biology Pages (based on Biology, 6th ed., 1996)]
7. ^ Sutovsky, P., et. al (Nov. 25, 1999). "Ubiquitin tag for sperm mitochondria". Nature 402: 371-372.
DOI:10.1038/46466 (http://dx.doi.org/10.1038/46466) . Discussed in Science News
(http://www.sciencenews.org/20000101/fob3.asp) .
8. ^ Mogensen, H. Lloyd (1996). "The Hows and Whys of Cytoplasmic Inheritance in Seed Plants".
American Journal of Botany 83: 383-404.
9. ^ Johns, D. R. (2003). "Paternal transmission of mitochondrial DNA is (fortunately) rare". Annals of
Neurology 54: 422-4.
General references
Cambell, Neil, et. al. (2006). Biology: concepts and connections. San Francisco: Benjamin Cummings.
ISBN 0-8053-7160-5.
National Center for Biotechnology Information (March 30, 2004). A Science primer
(http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html) (English). Retrieved on July 9, 2006.
Scheffler, I.E. (2001). "A century of mitochondrial research: achievements and perspectives".
Mitochondrion 1 (1): 3–31. PDF (http://www.mitoresearch.org/century.pdf)
External links
Arthropod mitochondira
(http://biology.plosjournals.org/perlserv/?request=get-document&doi=10%2E1371%2Fjournal%2Epbio%2E00401
Mitochondra Atlas (http://www.uni-mainz.de/FB/Medizin/Anatomie/workshop/EM/EMMitoE.html)
Mitochondria Research Portal (http://www.mitochondrial.net)
Mitochondria: Architecture dictates function (http://www.cytochemistry.net/Cell-biology/mitoch1.htm)
Mitochondira links (http://bama.ua.edu/~hsmithso/class/bsc_495/mito-plastids/mito_web.html)
Mitochondrion Reconstructed by Electron Tomography
(http://www.sci.sdsu.edu/TFrey/MitoMovie.htm)
Mitochondrion with Cell Biology (http://www.zytologie-online.net/mitochondrium.php)
Review of evidence addressing whether mitochondria form cellular networks or exist as discrete
organelles (http://www.the-elso-gazette.org/magazines/issue11/mreviews/mreviews1.asp)
Video Clip of Rat-liver Mitochondrion from Cryo-electron Tomography
(http://www.wadsworth.org/databank/electron/cryomito_dis2.html)
Information on Mitochondrial Diseases (http://www.circuitblue.com/mito)
Mitochondria and Aging (http://www.circuitblue.com/biogerontology/mito.shtml)
3D structures of proteins from inner mitochondrial membrane
(http://opm.phar.umich.edu/localization.php?localization=Mitochondrial%20inner%20membrane)
3D structures of proteins associated with outer mitochondrial membrane
(http://opm.phar.umich.edu/localization.php?localization=Mitochondrial%20outer%20membrane)
See also
7 of 8
23/11/06 15:06
Anti-mitochondrial antibodies
Chemiosmotic hypothesis
Chloroplast
Electrochemical potential
Endosymbiotic theory
Glycolysis
Mitochondrial disease
Mitochondrial DNA
Human mitochondrial genetics
Mitochondrial permeability transition pore
Submitochondrial particle
Wikimedia Commons has media related
to:
Mitochondrion
Organelles of the cell
Acrosome | Cell wall | Cell membrane | Chloroplast | Cilium/Flagellum | Centrosome | Cytoplasm | Endoplasmic reticulum |
Endosome | Golgi apparatus | Lysosome | Melanosome | Mitochondrion | Myofibril | Nucleus | Nucleolus (sub-organelle, found
within the nucleus) | Parenthesome | Peroxisome | Plastid | Ribosome | Vacuole | Vesicle
This article contains material from the Science Primer (http://www.ncbi.nlm.nih.gov/About/Primer)
published by the NCBI, which, as a US government publication, is in the public domain
Retrieved from "http://en.wikipedia.org/wiki/Mitochondrion"
Categories: Cellular respiration | Organelles
All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.
8 of 8
23/11/06 15:06
Reading(s) #3
Organelle - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Organelles
Organelle
(Redirected from Organelles)
In cell biology, an organelle is a discrete
structure of a cell having specialized functions.
There are many types of organelles,
particularly in the eukaryotic cells of higher
organisms. An organelle is to the cell what an
organ is to the body (hence the name
organelle, the suffix -elle being a diminutive).
Organelles were historically identified through
the use of microscopy, and were also identified
through the use of cell fractionation.
A few large organelles probably originated
from endosymbiont bacteria:
mitochondria (in almost all eukaryotes)
plastids (in plants and algae)
chloroplasts, mature forms of
etioplasts
chromoplasts
leucoplasts
amyloplasts
statoliths
elaioplasts
proteinoplasts
rhodoplasts
Schematic of typical animal cell, showing subcellular components.
Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5)
rough endoplasmic reticulum (ER) (6) Golgi apparatus (7)
Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11)
cytoplasm (12) lysosome (13) centrioles
Other organelles have had endosymbiotic origins suggested for them (notably flagella; see Evolution of
flagella), but these theories are not widely accepted.
Contents
1
2
3
4
Eukaryotic organelles
Prokaryotic organelles
See also
References
Eukaryotic organelles
Eukaryotes are the most structurally complex known cell type, and by definition are in part organized by
smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost
cell membrane. The larger organelles, such as the nucleus and vacuoles, are easily visible with moderate
magnification (although sometimes a clear view requires the application of chemicals that selectively stain
parts of the cells); they were among the first biological discoveries made after the invention of the
1 of 4
23/11/06 15:07
microscope.
Not all eukaryotic cells have all of the organelles listed below, and occasionally, exceptional species of cells
are missing organelles which might otherwise be considered universal to eukaryotic cells (such as
mitochondria). There are also occasional exceptions to the number of membranes surrounding organelles,
listed in the tables below (e.g. some which are listed as double-membraned are sometimes found with single
or triple membranes).
Major eukaryotic organelles
Organelle
Main function
Structure
Organisms
chloroplast
(plastid)
photosynthesis
double-membrane
compartment
plants,
protists
endoplasmic
reticulum
modification and folding of
new proteins and lipids
single-membrane
compartment
all
eukaryotes
Golgi apparatus
sorting and modification of
proteins
single-membrane
compartment
most
eukaryotes
mitochondrion
energy production
double-membrane
compartment
most
eukaryotes
vacuole
storage & homeostasis
single-membrane
compartment
eukaryotes
nucleus
DNA maintenance &
transcription to RNA
double-membrane
compartment
all
eukaryotes
Notes
has some
genes
has some
genes
has bulk of
genome
Organelles which have double-membranes and their own DNA are believed by many biologists of having
originally come from incompletely consumed or invading prokaryotic cells, which were adopted as a part of
the invaded cell through endosymbiosis.
Originally, the word organelle referred to large lipid bags within cells; later, as other cell parts were
discovered, the meaning was extended to also include smaller parts of cells.
Other eukaryotic organelles and cell components
Organelle
Main function
Organisms
acrosome
helps spermatoza fuse with
ovum
single-membrane
compartment
many animals
centriole
anchor for cytoskeleton
Microtubule protein
animals
cilium
movement in or of external
medium
Microtubule protein
animals, protists, few
plants
glyoxysome
conversion of fat into sugars
single-membrane
compartment
plants
double-membrane
compartment
a few unicellular
eukaryotes
energy & hydrogen
hydrogenosome production
2 of 4
Structure
lysosome
single-membrane
breakdown of large molecules compartment
melanosome
pigment storage
single-membrane
compartment
most eukaryotes
animals
23/11/06 15:07
mitosome
not characterized
double-membrane
compartment
a few unicellular
eukaryotes
myofibril
muscular contraction
bundled filaments
animals
nucleolus
ribosome production
protein-DNA-RNA
most eukaryotes
parenthesome
not characterized
not characterized
fungi
peroxisome
oxidation of protein
single-membrane
compartment
all eukaryotes
ribosome
translation of RNA into
proteins
RNA-protein
eukaryotes &
prokaryotes
vesicle
miscellaneous
single-membrane
compartment
all eukaryotes
Other related structures:
cytosol
endomembrane system
nucleosome
microtubule
cell membrane
Prokaryotic organelles
Prokaryotes are not as structurally complex as eukaryotes, and do not have any compartments enclosed by
lipid membranes. In the past they were often viewed as having little internal organization, but slowly details
are emerging about prokaryotic internal structures. One contributing discovery was that at least some
prokaryotes have microcompartments, which are compartments enclosed by proteins.
Prokaryotic organelles and cell components
Organelle
Main function
Structure
Organisms
carboxysome carbon fixation
protein-shell
compartment
some bacteria
flagellum
protein filament
some prokaryotes and
eukaryotes
inorganic crystal,
protein
magnetotactic bacteria
movement in external medium
magnetosome magnetic orientation
nucleoid
DNA maintenance & transcription
to RNA
DNA-protein
prokaryotes
plasmid
DNA exchange
circular DNA
some bacteria
ribosome
translation of RNA into proteins
RNA-protein
eukaryotes & prokaryotes
See also
Cell
Endosymbiotic theory
3 of 4
23/11/06 15:07
References
Alberts, Bruce et al. (2002). The Molecular Biology of the Cell, 4th ed., Garland Science, 2002, ISBN
0-8153-3218-1.
Kerfeld, Cheryl A et al., Protein Structures Forming the Shell of Primitive Bacterial Organelles,
Science 309:936-938 (5 August 2005).
Retrieved from "http://en.wikipedia.org/wiki/Organelle"
Category: Cell biology
All text is available under the terms of the GNU Free Documentation License. (See Copyrights for
details.)
4 of 4
23/11/06 15:07
Reading(s) #4
Cell Membranes Problem Set
http://www.biology.arizona.edu/cell_bio/problem_sets/membranes/i...
The Biology Project > Cell Biology > Cell Membranes > List of Problems
Cell Membranes Tutorial
This exercise introduces the dynamic complexes of proteins, carbohydrates,
and lipids that comprise cell membranes. You should learn that membranes are
fluid, with components that move, change, and perform vital physiological
roles as they allow cells to communicate with each other and their
environment. We also show that membranes also are important for regulating
ion and molecular traffic flow between cells,and that defects in membrane
components lead to many significant diseases.
Instructions: The following problems have multiple choice
answers. Correct answers are reinforced with a brief
explanation. Incorrect answers are linked to tutorials to help
solve the problem.
Overview
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Membrane components
Lipids and aqueous barriers
Hydrophobic forces
Osmosis
Membrane transport
Membrane proteins
Diffusion
Cotransport
Water flow solution
Membrane stability
Phospholipids
Penetrating lipid bilayer
Cell junctions
Energy requirements for transport
Oral rehydration
Membrane flow
The Biology Project > Cell Biology > Cell Membranes > List of Problems
Credits
The Biology Project
Department of Biochemistry and Molecular Biophysics
University of Arizona
May 2002
Revised: August 2004
Contact the Development Team
http://www.biology.arizona.edu
All contents copyright © 2002-04. All rights reserved.
1 of 1
31/03/07 06:01
Reading(s) #5
Cell nucleus - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Cell_nucleus
Cell nucleus
In cell biology, the nucleus (pl. nuclei; from
Latin nucleus or nuculeus, kernel) is a
membrane-enclosed organelle found in most
eukaryotic cells. It contains most of the cell's
genetic material, organized as multiple long linear
DNA molecules in complex with a large variety
of proteins such as histones to form
chromosomes. The genes within these
chromosomes make up the cell's nuclear genome.
The function of the nucleus is to maintain the
integrity of these genes and to control the
activities of the cell by regulating gene
expression.
Gene expression involves two major processes,
the first of which is transcription, in which DNA
The eukaryotic cell nucleus. Visible in this diagram are the
is used as a template to produce RNA. When this
ribosome-studded double membranes of the nuclear envelope, the
RNA encodes proteins, it is referred to as
DNA (complexed as chromatin), and the nucleolus. Within the
messenger RNA (mRNA). The second process is
cell nucleus is a viscous liquid called nucleoplasm, similar to the
called translation, and involves mRNA being used
cytoplasm found outside the nucleus.
as a template to produce proteins. Only
transcription occurs inside the nucleus, meaning
that all mRNA must be exported to the cytoplasm before it can be translated.
The main structural elements of the nucleus are the nuclear envelope, a double membrane that encloses the
entire organelle, and the nuclear lamina, a meshwork on the nuclear face of the envelope that adds
mechanical support. Because the membranes are impermeable to most molecules, nuclear pores are required
to allow movement across the envelope. These pores cross the entire envelope, providing a channel that
allows free movement of small solutes. The movement of larger molecules is controlled, and requires active
transport, facilitated by carrier proteins. Nuclear transport is of paramount importance to cell function, as
movement through the pores is required for both gene expression and chromosomal maintenance.
Although the interior of the nucleus does not contain any membrane-delineated bodies, its contents are not
uniform, and a number of subnuclear bodies exist, made up of unique proteins, RNA molecules, and DNA
conglomerates. The best known of these is the nucleolus, which is mainly involved in assembly of
ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they
translate mRNA.
The nucleus was the first organelle to be discovered, and was first described by Franz Bauer in 1802.[1] It
was later popularized by Scottish botanist Robert Brown in 1831. Brown was studying orchids
microscopically when he observed an opaque area, which he called the areola or nucleus, in the cells of the
flower's outer layer.[2]
1 of 13
23/11/06 15:09
Contents
1 Structure
1.1 Nuclear envelope and pores
1.2 Cytoskeleton
1.3 Chromosomes
1.4 Nucleolus
1.5 Other subnuclear bodies
1.5.1 Cajal bodies
1.5.2 Gemini of coiled bodies
1.5.3 Nemaline rods
1.5.4 PML bodies
1.5.5 Speckles
2 Function
2.1 Cell compartmentation
2.2 Gene expression
2.2.1 Processing of pre-mRNA
3 Dynamics and regulation
3.1 Nuclear transport
3.2 Assembly and disassembly
3.2.1 During the cell cycle
3.2.2 During apoptosis
3.2.3 During viral infection
4 Anucleated and polynucleated cells
5 Evolution
6 References
7 Further reading
8 External links
Structure
The nucleus is the largest cellular organelle.[3] It varies in diameter from 11 to 22 μm and occupies about
10% of the total cell volume.[4] The viscous liquid within it is called nucleoplasm, and is similar to the
cytoplasm found outside the nucleus.
Nuclear envelope and pores
2 of 13
23/11/06 15:09
The nuclear envelope consists of two cellular
membranes, an inner and an outer membrane,
arranged parallel to one another and separated by 10
to 50 nm. One of the features that make the nuclear
membranes unique are the large pores they contain.
The nuclear envelope encloses the nucleus and
separates the cell's genetic material from the
surrounding cytoplasm, serving as a barrier to
prevent macromolecules from diffusing freely
between the nucleoplasm and the cytoplasm.[5]
The outer nuclear membrane is continuous with the
membrane of the rough endoplasmic reticulum
(RER), and is similarly studded with ribosomes. The
space between the membranes is called the
perinuclear space and is continuous with the RER
lumen.
A cross section of a nuclear pore on the surface of the nuclear
envelope (1). Other diagram lables show (2) the outer ring,
(3) spokes, (4) basket, and (5) filaments.
Nuclear pores, which provide aqueous channels
through the envelope, are composed of a number of
different proteins, collectively referred to as nucleoporins. The pores are about 125 million daltons in
molecular weight and consist of around 50 (in yeast) to 100 proteins (in vertebrates).[3] The pores are 100
nm in diameter; however, after the annulus and other regulatory gating system molecules are present, the
space left for molecules to enter is reduced to 9 nm. This size allows the free passage of small water-soluble
molecules whilst excluding larger structures, such as DNA or proteins. Large molecules can still enter the
nucleus, but need to be transported. The nucleus of a typical mammalian cell will have about 3000 to 4000
pores throughout its envelope.[6]
Most proteins, ribosomal subunits, and some RNAs are transported through the pore complexes in a process
mediated by a family of transcription factors known as karyopherins. Those karyopherins that mediate
movement into the nucleus are also called importins, while those that mediate movement out of the nucleus
are called exportins. Most karyopherins interact directly with their cargo, although some use adaptor
proteins.[7] Steroid hormones such as cortisol and aldosterone, as well as other small lipid-soluble molecules
involved in intercellular signaling can diffuse through the cell membrane and into the cytoplasm, where they
bind nuclear receptor proteins that are trafficked into the nucleus. There they serve as transcription factors
when bound to their ligand; in the absence of ligand many such receptors function as histone deacetylases
that repress gene expression.[3]
Cytoskeleton
Two networks of intermediate filaments provide the nucleus with mechanical support: the nuclear lamina
forms an organised meshwork on the nuclear face of the envelope; less organised support is provided on the
cystolic face of the envelope. The mechanical functions provided include structural support for the nuclear
envelope, as well as providing anchorage sites for chromosomes and nuclear pores.[4]
The nuclear lamina is mostly composed of lamin proteins. The lamin proteins are transported into the
nucleus interior, where they are assembled, before being incorporated into the nuclear lamina.[8][9] In
addition to their role in the lamina, lamin proteins are also found inside the nucleoplasm where they form
3 of 13
23/11/06 15:09
another regular structure,[10] called the nucleoplasmic veil.
Like other intermediate filaments, the nuclear lamina monomer, the lamin, contains an alpha-helical region.
This domain is used by two monomers to coil around each other, facing the same direction, and form a
dimer structure called a coiled coil. Two of these dimer structures then join side by side, in an antiparallel
arrangement, to form a tetramer. This tetramer, composed of four lamin proteins, is called a protofilament.
Eight of these protofilaments form a lateral arrangement that is twisted to form a ropelike filament. These
filaments can be assembled or disassembled in a dynamic manner, meaning that changes in length depend on
the competing rates of filament addition and removal.[4]
Chromosomes
The cell nucleus contains the majority of the cell's genetic material, in the form of DNA molecules. During
most of the cell cycle these are organized in a DNA-protein complex known as chromatin, and during cell
division the chromatin can be seen to form well defined chromosomes.
There are two types of chromatin: euchromatin which is the less compact DNA form, and which contains
genes that are frequently expressed by the cell;[11] heterochromatin which is the more compact form, and
contains DNA that is not transcribed. It is further categorized into facultative heterochromatin, consisting of
those non-expressed genes, and constitutive heterochromatin, which consists of DNA's structural
components, telomeres and centromeres.
During interphase the chromatin organise themselves into discrete individual patches,[12] called chromosome
territories.[13] Active genes, which are generally found in the euchromatic region of the chromosome, tend
to be located towards the chromosome's territory boundary.[14]
Nucleolus
The nucleolus is a discrete densely-stained structure found in the
nucleus. It is not surrounded by a membrane, and is sometimes called a
suborganelle. It forms around tandem repeats of rDNA, DNA coding
for ribosomal RNA (rRNA). These regions are called nucleolar
organizer regions (NOR). The main roles of the nucleolus are to
synthesize rRNA and assemble ribosomes. The structural cohesion of
the nucleolus depends on its activity, as ribosomal assembly in the
nucleolus results in the transient association of nucleolar components,
facilitating further ribosomal assembly, and hence further association.
This model is supported by observations that inactivation of rDNA
results in intermingling of nucleolar structures.[15]
An electron micrograph of a cell
nucleus, showing the darkly stained
nucleolus.
The first step in ribosomal assembly is transcription of the rDNA, by a
protein called RNA polymerase I, forming a large pre-rRNA precursor.
This is cleaved into the subunits 5.8S, 18S, and 28S rRNA.[16] The
transcription, post-transcriptional processing, and assembly of rRNA
occurs in the nucleolus, aided by small nucleolar RNA (snoRNA) molecules, some of which are derived
from spliced introns from messenger RNAs encoding genes related to ribosomal function. The assembled
ribosomal subunits are the largest structures passed through the nuclear pores.[3]
4 of 13
23/11/06 15:09
When observed under the electron microscope, the nucleolus can be seen to consist of three distinguishable
regions: the inner most fibrillar centers (FCs), surrounded by the dense fibrillar component (DFC), which in
turn is bordered by the granular component (GC). Transcription of the rDNA occurs either in the FC or at
the FC-DFC boundary, and therefore when rDNA transcription in the cell is increased more FCs are detected.
Most of the cleavage and modification of rRNAs occurs in the DFC, while the latter steps involving protein
assembly onto the ribosomal subunits occurs in the GC.[16]
Other subnuclear bodies
Besides the nucleolus, the nucleus contains a number of other
non-membrane delineated bodies. These include Cajal bodies,
Gemini of coiled bodies, paraspeckles, polymorphic interphase
karyosomal association (PIKA), PML bodies, and speckles.
Although little is known about a number of these domains,
they are significant in that they show that the nucleoplasm is
not uniform mixture, but rather contains organised functional
subdomains.[19]
Cajal bodies
Subnuclear structure sizes
Structure name
Structure diameter
Cajal bodies
0.2-2.0 μm [17]
PIKA
5 μm [18]
PML bodies
0.2-1.0 μm [19]
Speckles
20–25 nm[18]
A nucleus will contain between 1 and 10 compact structures called Cajal bodies (CB). The diameter of which
measure between 0.2 μm and 2.0 μm depending on the cell type and species,[17] and when seen under an
electron microscope, appear as balls of tangled thread.[18] They are involved in a number of different roles
relating to RNA processing, specifically small nucleolar RNA (snoRNA) and small nuclear RNA (snRNA)
maturation, and histone mRNA modification.[17]
Gemini of coiled bodies
Similar to Cajal bodies are Gemini of coiled bodies, also called Gems. The name is derived from the Gemini
constellation, the twins, in reference to Gems' close association with CBs. They are similar in size and shape
to CBs, but differ in composition. Unlike CBs, Gems don't contain snRNPs, but do contain a protein called
survivor of motor neurons (SMN) whose function relates to snRNP biogenesis. Gems are believed to assist
CBs in snRNP biogenesis.[20]
Nemaline rods
The presence of small intranuclear rods have been reported in some cases of nemaline myopathy. This
condition typically results from mutations in actin, and the rods themsleves consist of mutant actin as well as
other cytoskeletal proteins.[21]
PML bodies
Promyelocytic leukaemia bodies (PML bodies) are spherical bodies found scattered throughout the
nucleoplasm, measuring around 0.2-1.0 μm. They are known by a number of other names, including
nuclear domain 10 (ND10), Kremer bodies, and PML oncogenic domains. They are often seen in the
nucleus in association with Cajal bodies and cleavage bodies. It has been suggested that they play a role in
regulating transcription.[19]
5 of 13
23/11/06 15:09
Speckles
Sometimes referred to as interchromatin granule clusters, speckles are rich in splicing snRNPs and other
splicing proteins necessary for pre-mRNA processing. Because of a cell's changing requirements, the
composition and location of these bodies changes according to mRNA transcription and regulation via
phosphorylation of specific proteins.[22]
Function
The main functions of the nucleus are providing a compartment separated from the rest of the cell and
controlling transcription.
Cell compartmentation
The nuclear envelope allows the nucleus to control its contents, and separate them from the rest of the
cytoplasm where necessary. This is important for controlling processes on either side of the nuclear
membrane. For example:
In some cases where a cytoplasmic process needs to be restricted, a key participant is removed to the nucleus.
For example this occurs in the case of glycolysis, a cellular pathway for breaking down glucose to produce
energy. Hexokinase is an enzyme responsible for the first the step of glycolysis, forming
glucose-6-phosphate from glucose. At high concentrations of fructose-6-phosphate, a molecule made later
from glucose-6-phosphate, a regulator protein removes hexokinase to the nucleus.[23]
In order to control which genes are being transcribed, the cell separates the proteins controlling gene
expression from those genes. For example in the case of NF-κB genes, which are involved in most
inflammatory responses, the genes are transcribed in response to a signal pathway. In one of the pathways
involving these genes, TNF-α binds to a cell membrane receptor resulting in the recruitment of signalling
proteins, and eventually freeing NF-κB. The nuclear localisation signal allows NF-κB to be transported
through the nuclear pore and into the nucleus where it stimulates the transcription of the target genes.[4]
The compartmentation allows the cell to prevent translation of unspliced mRNA.[24] Eukaryotic mRNA
contains introns that must be removed before being translated to produce functional proteins. The splicing is
done inside the nucleus before the mRNA can be accessed by ribosomes for translation. Without the nucleus
ribosomes would translate newly transcribed (unprocessed) mRNA resulting in misformed proteins.
6 of 13
23/11/06 15:09
Gene expression
Gene expression first involves transcription, in which DNA is used as a
template to produce RNA. In the case of genes encoding proteins, that
RNA produced from this process is messenger RNA (mRNA), which
then needs to be translated by ribosomes to form a protein. As
ribosomes are located outside the nucleus, mRNA produced needs to be
exported.
A micrograph of ongoing gene
transcription of ribosomal RNA
illustrating the growing primary
transcripts. "Begin" indicates the 5'
end of the DNA, where new RNA
synthesis begins; "end" indicates
the 3' end, where the primary
transcripts are almost complete.
Since the nucleus is the site of transcription, it also contains a variety of
proteins which either directly mediate transcription or are involved in
regulating the process. These proteins include helicases that unwind the
double-stranded DNA molecule to facilitate access to it, RNA
polymerases that synthesize the growing RNA molecule, topoisomerases
that restore the supercoiled state of DNA after it has been unwound, and
a large variety of transcription factors that regulate expression.
Processing of pre-mRNA
Newly synthesized mRNA molecules are known as primary transcripts
or pre-mRNA. They must undergo post-transcriptional modification in
the nucleus before being exported to the cytoplasm; mRNA that appears
in the nucleus without these modifications is degraded rather than used
for protein translation. The three main modifications are 5' capping, 3' polyadenylation, and RNA splicing.
While in the nucleus, pre-mRNA is associated with a variety of proteins in complexes known as
heterogeneous ribonucleoprotein particles (hnRNPs). Addition of the 5' cap occurs co-transcriptionally and
is the first step in post-translational modification. The 3' poly-adenine tail is only added after transcription is
complete.
RNA splicing, carried out by a complex called the spliceosome, is the process by which introns, or regions of
DNA that do not code for protein, are removed from the pre-mRNA and the remaining exons connected to
re-form a single continuous molecule. This process normally occurs after 5' capping and 3' polyadenylation
but can begin before synthesis is complete in transcripts with many exons.[3] Many pre-mRNAs, including
those encoding antibodies, can be spliced in multiple ways to produce different mature mRNAs that encode
different protein sequences. This process is known as alternative splicing, and allows production of a large
variety of proteins from a limited amount of DNA.
Dynamics and regulation
The nucleus is a dynamic structure that changes according the cell's requirements. In order to control the
nuclear functions, the processes of entry and exit from the nucleus are regulated.
Nuclear transport
7 of 13
23/11/06 15:09
The entry and exit of large molecules from the
nucleus is tightly controlled by the nuclear pore
complexes. Although small molecules can enter
the nucleus without regulation,[25]
macromolecules such as RNA and proteins
require association karyopherins called
importins to enter the nucleus and exportins to
exit. "Cargo" proteins that must be translocated
from the cytoplasm to the nucleus contain short
amino acid sequences known as nuclear
localization signals which are bound by
importins, while those transported from the
nucleus to the cytoplasm carry nuclear export
signals bound by exportins. The ability of
Macromolecules, such as RNA and proteins, are actively
importins and exportins to transport their cargo
transported across the nuclear membrane in a process called the
is regulated by GTPases, enzymes that
Ran-GTP nuclear transport cycle.
hydrolyze the molecule guanosine triphosphate
to release energy. The key GTPase in nuclear
transport is Ran, which can bind either GTP or GDP (guanosine diphosphate) depending on whether it is
located in the nucleus or the cytoplasm. Whereas importins depend on RanGTP to dissociate from their
cargo, exportins require RanGTP in order to bind to their cargo.[7]
Nuclear import depends on the importin binding its cargo in the cytoplasm, carrying it through the nuclear
pore into the nucleus. Inside the nucleus RanGTP acts to separate the cargo from the importin, allowing the
importin to exit the nucleus and be reused. Nuclear export is similar, as the exportin binds the cargo inside
the nucleus, leaves through the nuclear pore, and by interacting with RanGDP separates.
Specialized export proteins exist for translocation of mature mRNA and tRNA to the cytoplasm after
post-transcriptional modification is complete. This quality-control mechanism is important due to the these
molecules' central role in protein translation; mis-expression of a protein due to incomplete excision of exons
or mis-incorporation of amino acids could have negative consequences for the cell; thus incompletely
modified RNA that reaches the cytoplasm is degraded rather than used in translation.[3]
Assembly and disassembly
During its lifetime a nucleus may be broken down, a process which depending on the circumstances may
eventually be followed by its being reconstructed. During these events, the main components whose break
down needs to be controlled are the structural ones, namely the nuclear envelope and the nuclear lamina.
During the cell cycle
8 of 13
23/11/06 15:09
During the cell cycle the cell divides to form two cells. In order for
this process to be possible, each of the new daughter cells must
have a full set of genes, a process requiring replication of the
chromosomes as well as segregation of the separate sets. This
requires that the replicated chromosomes, the sister chromatids, be
attached to microtubules, which in turn are attached to different
centrosomes. The sister chromatids can then be pulled to separate
locations in the cell. However, in many cells the centrosome is
located in the cytoplasm, outside the nucleus, the microtubles
would be unable to attach to the chromatids in the presence of the
An image of a newt lung cell stained with
fluorescent dyes during anaphase. The
mitotic spindles can be seen, stained
green, attached to the two sets of
chromosomes, stained light blue. Each
complete set of chromosomes is being
pulled by the spindle towards its own
centrosomes, also stained green.
nuclear envelope.[26] Therefore the early stages in the cell cycle,
beginning in prophase and until around prometaphase, the nuclear
membrane is dismantled.[10] Likewise, during the same period, the
nuclear lamina is also dissembled, a process regulated by
phosphoyrlation of the lamins.
Towards the end of the cell cycle, the nuclear membrane is
reformed, and around the same time, the nuclear lamina
reassembled by dephosphorylating the lamins.[10]
During apoptosis
Apoptosis is a controlled process resulting in death of the cell. Various of the changes directly affect the
nucleus and its contents, especially condensation of the chromatin, disintegration of the nuclear envelope
and lamina. The progressive organisation of the nuclear lamina throughout apoptosis is used to initiate and
regulate the various phases of apoptosis.[10] The breakdown of the lamina is controlled by a group of
proteins called caspases that cleave the individual lamins.
During viral infection
The nuclear envelope acts as a barrier that prevents both DNA and RNA viruses from entering the nucleus.
Some viruses require access to proteins inside the nucleus in order to replicate and/or assemble. DNA viruses,
such as herpesvirus replicate and assemble in the cell nucleus, and exit by budding through the inner nuclear
membrane. This process is accompanied by disassembly of the lamina on the nuclear face of the inner
membrane.[10]
Anucleated and polynucleated cells
9 of 13
23/11/06 15:09
Although most cells have a single nucleus, some cell types can have none or
many nuclei. This can be a normal process, as in the maturation of mammalian
red blood cells, or an abnormal product of faulty cell division.
Anucleated cells contain no nucleus and are therefore incapable of dividing to
produce daughter cells. The best-known anucleated cell is the mammalian red
blood cell, or erythrocyte, which also lacks other organelles such as mitochondria
and serves primarily as a transport vessel to ferry oxygen from the lungs to the
body's tissues. Erythrocytes mature via erythropoiesis in the bone marrow, where
they lose their nuclei, organelles, and ribosomes. The nucleus is expelled during
the process of differentiation from an erythroblast to a reticulocyte, the
immediate precursor of the mature erythrocyte.[27] The presence of mutagens
may induce the release of some immature "micronucleated" erythrocytes into the
bloodstream.[28][29] Anucleated cells can also arise from flawed cell division in
which one daughter lacks a nucleus and the other is binucleate.
Human red blood cells,
like those of other
mammals, lack nuclei.
This occurs as a normal
part of the cells'
development.
Polynucleated cells contain multiple nuclei. Most Acantharean species of
protozoa[30] and some fungi in mycorrhizae[31] have naturally polynucleated cells. Cells arising from the
fusion of monocytes and macrophages, known as giant multinucleated cells, sometimes accompany
inflammation[32] and are also implicated in tumor formation.[33]
Evolution
There are four contending theories of the origin of the nucleus in eukaryotic cells.[34]
One theory proposed by Purificacion Lopez-Garcia and David Moreira is the syntrophic model. This model
holds that a symbiotic relationship between the archaea and bacteria created the nucleus containing
eukaryotic cell. It is believed that archaea, similar to modern methanogenic archaea, entered bacteria, similar
to modern myxobacteria, and developed a symbiosis which eventually fused into a chimeric new organism
the eukarya. An archaeal origin of the nucleus is supported by observations that archaea and eukarya have
similar genes for certain proteins, including histones. Observations that myxobacteria are motile, can form
multicelluar complexes, and possess kinases and G proteins similar to eukarya, supports a bacterial origin for
the eukaryotic cell.[35]
Another model by John Fuerst proposes that eukaryotic like cells existed at the same time as archaea and
bacteria were splitting off on their own lineages. This model is based on the existence of modern
planctomycetes bacteria that possess a nuclear structure with primitive pores and other compartmentalized
membrane structures.[36] A similar proposal states that a eukarya like cell, the chronocyte, evolved first and
phagocytosed archaea and bacteria to generate the nucleus and the eukaryotic cell.[37]
The third and most controversial model, known as viral eukaryogenesis, posits that the membrane-bound
nucleus, along with other eukaryotic features, originated from the infection of a prokaryote by a virus. The
suggestion is based on similarities between eukaryotes and viruses such as linear DNA strands, mRNA
capping, and tight binding to proteins (analogizing histones to viral envelopes). One version of the proposal
suggests that the nucleus evolved in concert with phagocytosis to form an early cellular "predator".[38]
Another variant proposes that eukaryotes originated from early archaea infected by poxviruses, on the basis
of observed similarity between the DNA polymerases in modern poxviruses and eukaryotes.[39][40] It has
been suggested that the as-yet-unresolved question of the evolution of sex could be related to the viral
10 of 13
23/11/06 15:09
eukaryogenesis hypothesis.[41]
The fourth model proposed by Albert de Roos is based on the self assembly of membranes by lipid-protein
interactions. The model is based on a Darwinist approach of the evolution of compartmentalization of
function by interaction of laminar proteins, intrinsic proteins, and cytoskeletal proteins with lipid vesicles to
form the nucleus, and exomembranes to form the endoplasmic reticulum and plasma membrane.[42]
References
1 . ^ Harris, H (1999). The Birth of the Cell. New Haven: Yale 13.
University Press.
2 . ^ Brown, Robert (1866). "On the Organs and Mode of
Fecundation of Orchidex and Asclepiadea". Miscellaneous
14.
Botanical Works I: 511-514.
3 . ^ a b c d e f Lodish, H, Berk A, Matsudaira P, Kaiser CA,
Krieger M, Scott MP, Zipursky SL, Darnell J. (2004).
Molecular Cell Biology, 5th, New York: WH Freeman.
4 . ^ a b c d (2002) Bruce Alberts, Alexander Johnson, Julian
Lewis, Martin Raff, Keith Roberts, Peter Walter Molecular
15.
Biology of the Cell, 4th, Garland Science.
5 . ^ Paine PL, Moore LC, Horowitz SB. Nuclear envelope
permeability. Nature. 1975 Mar 13;254(5496):109-14.
PMID 1117994
6 . ^ (1996) “Ch3”, Rodney Rhoades, Richard Pflanzer Human 16.
Physiology, 3rd, Saunders College Publishing.
7 . ^ a b Pemberton, Lucy F., Bryce M. Paschal (2005).
"Mechanisms of Receptor-Mediated Nuclear Import and
17.
Nuclear Export". Traffic 6: 187-198.
DOI:10.1111/j.1600-0854.2005.00270.x
(http://dx.doi.org/10.1111/j.1600-0854.2005.00270.x) .
8 . ^ Stuurman, Nico, Susanne Heins and Ueli Aebi (1998).
"Nuclear Lamins: Their Structure, Assembly, and
Interactions". Journal of Strucutral Biology (122): 42-66.
18.
PMID 9724605.
9 . ^ Goldman, A.E., Moir, R. D., Montag, L. M., Stewart,
19.
M., and Goldman, R. D (1992). "Pathway of incorporation
of microinjected lamin A into the nuclear envelope".
Journal of Cell Biology (119): 725-732. PMID 1429833.
20.
10. ^ a b c d e Goldman, Robert D., Yosef Gruenbaum, Robert
D. Moir, Dale K. Shumaker and Timothy P. Spann (2002).
"Nuclear lamins: building blocks of nuclear architecture".
21.
Genes & Dev. (16): 533-547. DOI:10.1101/gad.960502
(http://dx.doi.org/10.1101/gad.960502) .
11. ^ Ehrenhofer-Murray, Ann E. (June 2004). "Chromatin
dynamics at DNA replication, transcription and repair".
22.
European Journal of Biochemistry 271 (12): 2335.
DOI:10.1111/j.1432-1033.2004.04162.x
(http://dx.doi.org/10.1111/j.1432-1033.2004.04162.x) .
12. ^ Schardin, Margit, T. Cremer, H. D. Hager, M. Lang (Dec
1985). "Specific staining of human chromosomes in
23.
Chinese hamster x man hybrid cell lines demonstrates
interphase chromosome territories
(http://www.springerlink.com/content/lv101t8w17306071/)24.
". Human Genetics 71 (4): 281-287.
DOI:10.1007/BF00388452
(http://dx.doi.org/10.1007/BF00388452) . PMID
25.
2416668.
11 of 13
^ Lamond, Angus I., William C. Earnshaw (24 April
1998). "Structure and Function in the Nucleus". Science
280: 547-553. PMID 9554838.
^ Kurz, A, S Lampel, JE Nickolenko, J Bradl, A Benner,
RM Zirbel, T Cremer and P Lichter (1996). "Active and
inactive genes localize preferentially in the periphery of
chromosome territories
(http://intl.jcb.org/cgi/content/abstract/135/5/1195) ".
The Journal of Cell Biology 135: 1195-1205. PMID
8947544.
^ Hernandez-Verdun, Danie`le (2006). "Nucleolus: from
structure to dynamics". Histochem. Cell. Biol (125):
127-137. DOI:10.1007/s00418-005-0046-4
(http://dx.doi.org/10.1007/s00418-005-0046-4) .
^ a b Lamond, Angus I., Judith E. Sleeman. "Nuclear
substructure and dynamics". Current Biology 13 (21):
R825-8. PMID 14588256.
^ a b c Cioce, Mario, William C. Earnshaw (2005). "Cajal
Bodies: A Long History of Discovery". Annual Review of
Cell and Developmental Biology 21: 105-131.
DOI:0.1146/annurev.cellbio.20.010403.103738
(http://dx.doi.org/0.1146/annurev.cellbio.20.010403.103738
.
^ a b c Pollard, Thomas D., William C. Earnshaw (2004).
Cell Biology. Philadelphia: Saunders. ISBN 0721633609.
^ a b c Dundr, Miroslav, Tom Misteli (2001). "Functional
architecture in the cell nucleus". Biochem. J. (356):
297-310. PMID 11368755.
^ Matera, A. Gregory (1998). "Of Coiled Bodies, Gems,
and Salmon". Journal of Cellular Biochemistry (70):
181–192. PMID 9671224.
^ Goebel, H.H., I Warlow (January 1997). "Nemaline
myopathy with intranuclear rods--intranuclear rod
myopathy". Neuromuscular Disorders 7 (1): 13-19. PMID
9132135.
^ Handwerger, Korie E., Joseph G. Gall (January 2006).
"Subnuclear organelles: new insights into form and
function". TRENDS in Cell Biology 16 (1): 19-26.
DOI:10.1016/j.tcb.2005.11.005
(http://dx.doi.org/10.1016/j.tcb.2005.11.005) .
^ Lehninger, Albert L., David L. Nelson, Michael M. Cox.
(2000). Lehninger principles of biochemistry, 3rd, New
York: Worth Publishers. ISBN 1572599316.
^ Görlich, Dirk, Ulrike Kutay (1999). "Transport between
the cell nucleus and the cytoplasm". Ann. Rev. Cell Dev.
Biol. (15): 607-660. PMID 10611974.
^ Watson, JD, Baker TA, Bell SP, Gann A, Levine M,
Losick R. (2004). “Ch9-10”, Molecular Biology of the
23/11/06 15:09
Gene, 5th ed., Peason Benjamin Cummings; CSHL Press..
26. ^ Lippincott-Schwartz, Jennifer (7 March 2002). "Cell
biology: Ripping up the nuclear envelope". Nature 416
(6876): 31-32. DOI:10.1038/416031a
(http://dx.doi.org/10.1038/416031a) .
27. ^ Skutelsky, E., Danon D. (June 1970). "Comparative
study of nuclear expulsion from the late erythroblast and
cytokinesis". J Cell Biol (60(3)): 625-35. PMID
5422968.
28. ^ Torous, DK, Dertinger SD, Hall NE, Tometsko CR.
(2000). "Enumeration of micronucleated reticulocytes in
rat peripheral blood: a flow cytometric study". Mutat Res
(465(1-2)): 91-9. PMID 10708974.
29. ^ Hutter, KJ, Stohr M. (1982). "Rapid detection of
mutagen induced micronucleated erythrocytes by flow
cytometry". Histochemistry (75(3)): 353-62. PMID
7141888.
30. ^ Zettler, LA, Sogin ML, Caron DA (1997). "Phylogenetic
relationships between the Acantharea and the
Polycystinea: A molecular perspective on Haeckel's
Radiolaria". Proc Natl Acad Sci USA (94): 11411-11416.
PMID 9326623.
31. ^ Horton, TR (2006). "The number of nuclei in
basidiospores of 63 species of ectomycorrhizal
Homobasidiomycetes". Mycologia (98(2)): 233-8. PMID
16894968.
32. ^ McInnes, A, Rennick DM (1988). "Interleukin 4 induces
cultured monocytes/macrophages to form giant
multinucleated cells". J Exp Med (167): 598-611. PMID
3258008.
33. ^ Goldring, SR, Roelke MS, Petrison KK, Bhan AK
(1987). "Human giant cell tumors of bone identification
and characterization of cell types". J Clin Invest (79(2)):
483–491. PMID 3027126.
34. ^ Pennisi E. (2004). "Evolutionary biology. The birth of
the nucleus". Science 305 (5685): 766-8.. PMID
15297641.
35. ^ Lopez-Garcia P, Moreira D. (2006). "Selective forces for
the origin of the eukaryotic nucleus". Bioessays 28 (5):
525-33. PMID 16615090.
36. ^ Fuerst JA. (2005). "Intracellular compartmentation in
planctomycetes.journal = Annu Rev Microbiol.." 59:
299-328. PMID 15910279.
37. ^ Hartman H, Fedorov A. (2002). "The origin of the
eukaryotic cell: a genomic investigation". Proc Natl Acad
Sci U S A. 99 (3): 1420-5. PMID 11805300.
38. ^ Bell PJ. (2001). Viral eukaryogenesis: was the ancestor
of the nucleus a complex DNA virus? J Mol Biol
Sep;53(3):251-6.
39. ^ Takemura M. (2001). Poxviruses and the origin of the
eukaryotic nucleus. J Mol Evol 52(5):419-25.
40. ^ Villareal LP, DeFilippis VR. (2000). A hypothesis for
DNA viruses as the origin of eukaryotic replication
proteins. J Virol 74(15):7079-84.
41. ^ Bell PJ. (2006). Sex and the eukaryotic cell cycle is
consistent with a viral ancestry for the eukaryotic nucleus.
J Theor Biol Epub before print.
42. ^ de Roos AD (2006). "The origin of the eukaryotic cell
based on conservation of existing interfaces". Artif Life
12 (4): 513-23.. PMID 16953783.
Further reading
Goldman, Robert D., Yosef Gruenbaum, Robert D. Moir, Dale K. Shumaker and Timothy P. Spann
(2002). "Nuclear lamins: building blocks of nuclear architecture". Genes & Dev. (16): 533-547.
DOI:10.1101/gad.960502 (http://dx.doi.org/10.1101/gad.960502) .
A review article about nuclear lamins, explaining their structure and various roles
Görlich, Dirk, Ulrike Kutay (1999). "Transport between the cell nucleus and the cytoplasm". Ann. Rev.
Cell Dev. Biol. (15): 607-660. PMID 10611974.
A review article about nuclear transport, explains the principles of the mechanism, and the various
transport pathways
Lamond, Angus I., William C. Earnshaw (24 APRIL 1998). "Structure and Function in the Nucleus".
Science 280: 547-553. PMID 9554838.
A review article about the nucleus, explaining the structure of chromosomes within the organelle, and
describing the nucleolus and other subnuclear bodies
Pennisi E. (2004). "Evolutionary biology. The birth of the nucleus". Science 305 (5685): 766-8..
PMID 15297641.
A review article about the evolution of the nucleus, explaining a number of different theories
Pollard, Thomas D., William C. Earnshaw (2004). Cell Biology. Philadelphia: Saunders. ISBN
0721633609.
12 of 13
23/11/06 15:09
A university level textbook focusing on cell biology. Contains information on nucleus structure and
function, including nuclear transport, and subnuclear domains
External links
cellnucleus.com (http://www.cellnucleus.com/education_main.htm) Website covering structure and
function of the nucleus from the Department of Oncology at the University of Alberta.
The Nuclear Protein Database (http://npd.hgu.mrc.ac.uk/compartments.html) Information on nuclear
components.
The Nucleus Collection (http://cellimages.ascb.org/cdm4/browse.php?CISOROOT=/p4041coll6) in the
Image & Video Library (http://cellimages.ascb.org/) of The American Society for Cell Biology
(http://www.ascb.org/) contains peer-reviewed still images and video clips that illustrate the nucleus.
Nuclear Envelope and Nuclear Import Section (http://cellimages.ascb.org/u?/p4041coll11,62) from
Landmark Papers in Cell Biology
(http://cellimages.ascb.org/cdm4/browse.php?CISOROOT=%2Fp4041coll11) , Joseph G. Gall, J.
Richard McIntosh, eds., contains digitized commentaries and links to seminal research papers on the
nucleus. Published online in the Image & Video Library (http://cellimages.ascb.org/) of The American
Society for Cell Biology (http://www.ascb.org/)
Retrieved from "http://en.wikipedia.org/wiki/Cell_nucleus"
Categories: Organelles | Medical terms
details.)
13 of 13
23/11/06 15:09
Reading(s) #6
Genetics/Introduction - Wikibooks, collection of open-content textbooks
http://en.wikibooks.org/wiki/Genetics/Introduction
Genetics/Introduction
< Genetics
Genetics, study of the function and behavior of genes. Genes are bits of biochemical instructions found
inside the cells of every organism from bacteria to humans. Offspring receive a mixture of genetic
information from both parents. This process contributes to the great variation of traits that we see in nature,
such as the color of a flower’s petals, the markings on a butterfly’s wings, or such human behavioral traits as
personality or musical talent. Geneticists seek to understand how the information encoded in genes is used
and controlled by cells and how it is transmitted from one generation to the next. Geneticists also study how
tiny variations in genes can disrupt an organism’s development or cause disease. Increasingly, modern
genetics involves genetic engineering, a technique used by scientists to manipulate genes. Genetic
engineering has produced many advances in medicine and industry, but the potential for abuse of this
technique has also presented society with many ethical and legal controversies.
Genetic information is encoded and transmitted from generation to generation in deoxyribonucleic acid
(DNA). DNA is a coiled molecule organized into structures called chromosomes within cells. Segments along
the length of a DNA molecule form genes. Genes direct the synthesis of proteins, the molecular laborers that
carry out all life-supporting activities in the cell. Although all humans share the same set of genes,
individuals can inherit different forms of a given gene, making each person genetically unique.
Since the earliest days of plant and animal domestication, around 10,000 years ago, humans have
understood that characteristic traits of parents could be transmitted to their offspring. The first to speculate
about how this process worked were Greek scholars around the 4th century bc, who promoted theories based
on conjecture or superstition. Some of these theories remained in favor for several centuries. The scientific
study of genetics did not begin until the late 19th century. In experiments with garden peas, Austrian monk
Gregor Mendel described the patterns of inheritance, observing that traits were inherited as separate units.
These units are now known as genes. Mendel’s work formed the foundation for later scientific achievements
that heralded the era of modern genetics.
Most of the discoveries about genes and DNA happened after Charles Darwin 1859 famous book on The
Origin of Species by Means of Natural Selection, or the preservation of favoured races in the struggle for
life.
Retrieved from "http://en.wikibooks.org/wiki/Genetics/Introduction"
Category: Genetics
details).
Privacy policy
About Wikibooks
Disclaimers
1 of 1
23/11/06 15:09
Reading(s) #7
Reading(s)
#7.1
Intro to Genetics
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookgenint...
INTRODUCTION TO GENETICS
Table of Contents
Heredity, historical perspectives | The Monk and his peas | Principle of segregation
Dihybrid Crosses | Mutations | Genetic Terms | Links
Heredity, Historical Perspective | Back to Top
For much of human history people were unaware of the scientific details of how
babies were conceived and how heredity worked. Clearly they were conceived, and
clearly there was some hereditary connection between parents and children, but the
mechanisms were not readily apparent. The Greek philosophers had a variety of ideas:
Theophrastus proposed that male flowers caused female flowers to ripen; Hippocrates
speculated that "seeds" were produced by various body parts and transmitted to
offspring at the time of conception, and Aristotle thought that male and female semen
mixed at conception. Aeschylus, in 458 BC, proposed the male as the parent, with the
female as a "nurse for the young life sown within her".
During the 1700s, Dutch microscopist Anton van Leeuwenhoek (1632-1723)
discovered "animalcules" in the sperm of humans and other animals. Some scientists
speculated they saw a "little man" (homunculus) inside each sperm. These scientists
formed a school of thought known as the "spermists". They contended the only
contributions of the female to the next generation were the womb in which the
homunculus grew, and prenatal influences of the womb. An opposing school of
thought, the ovists, believed that the future human was in the egg, and that sperm
merely stimulated the growth of the egg. Ovists thought women carried eggs
containing boy and girl children, and that the gender of the offspring was determined
well before conception.
Pangenesis was an idea that males and females formed "pangenes" in every organ.
These pangenes subsequently moved through their blood to the genitals and then to the
children. The concept originated with the ancient Greeks and influenced biology until
little over 100 years ago. The terms "blood relative", "full-blooded", and "royal
blood" are relicts of pangenesis. Francis Galton, Charles Darwin's cousin,
experimentally tested and disproved pangenesis during the 1870s.
Blending theories of inheritance supplanted the spermists and ovists during the 19th
century. The mixture of sperm and egg resulted in progeny that were a "blend" of two
parents' characteristics. Sex cells are known collectively as gametes (gamos, Greek,
meaning marriage). According to the blenders, when a black furred animal mates with
white furred animal, you would expect all resulting progeny would be gray (a color
intermediate between black and white). This is often not the case. Blending theories
ignore characteristics skipping a generation. Charles Darwin had to deal with the
implications of blending in his theory of evolution. He was forced to recognize
blending as not important (or at least not the major principle), and suggest that science
1 of 13
23/11/06 15:10
Intro to Genetics
of the mid-1800s had not yet got the correct answer. That answer came from a
contemporary, Gregor Mendel, although Darwin apparently never knew of Mendel's
work.
The Monk and his peas | Back to Top
An Austrian monk, Gregor Mendel, developed the fundamental principles that would
become the modern science of genetics. Mendel demonstrated that heritable properties
are parceled out in discrete units, independently inherited. These eventually were
termed genes.
Gregor Mendel, the Austrian monk who figured out the rules of hereity. The above photo
is from http://www.open.cz/project/tourist/person/photo.htm.
Mendel reasoned an organism for genetic experiments should have:
1. a number of different traits that can be studied
2. plant should be self-fertilizing and have a flower structure that limits accidental
contact
3. offspring of self-fertilized plants should be fully fertile.
Mendel's experimental organism was a common garden pea (Pisum sativum), which
has a flower that lends itself to self-pollination. The male parts of the flower are
termed the anthers. They produce pollen, which contains the male gametes (sperm).
The female parts of the flower are the stigma, style, and ovary. The egg (female
gamete) is produced in the ovary. The process of pollination (the transfer of pollen
from anther to stigma) occurs prior to the opening of the pea flower. The pollen grain
grows a pollen tube which allows the sperm to travel through the stigma and style,
eventually reaching the ovary. The ripened ovary wall becomes the fruit (in this case
the pea pod). Most flowers allow cross-pollination, which can be difficult to deal with
in genetic studies if the male parent plant is not known. Since pea plants are
self-pollinators, the genetics of the parent can be more easily understood. Peas are also
self-compatible, allowing self-fertilized embryos to develop as readily as out-fertilized
embryos. Mendel tested all 34 varieties of peas available to him through seed dealers.
The garden peas were planted and studied for eight years. Each character studied had
two distinct forms, such as tall or short plant height, or smooth or wrinkled seeds.
2 of 13
23/11/06 15:10
Intro to Genetics
Mendel's experiments used some 28,000 pea plants.
3 of 13
23/11/06 15:10
Intro to Genetics
Some of Mendel's traits as expressed in garden peas. Images from Purves et al., Life: The
Mendel's contribution was unique because of his methodical approach to a definite
problem, use of clear-cut variables and application of mathematics (statistics) to the
problem. Gregor Using pea plants and statistical methods, Mendel was able to
demonstrate that traits were passed from each parent to their offspring through the
inheritance of genes.
Mendel's work showed:
4 of 13
23/11/06 15:10
Intro to Genetics
1. Each parent contributes one factor of each trait shown in offspring.
2. The two members of each pair of factors segregate from each other during
gamete formation.
3. The blending theory of inheritance was discounted.
4. Males and females contribute equally to the traits in their offspring.
5. Acquired traits are not inherited.
Principle of Segregation | Back to Top
Mendel studied the inheritance of seed shape first. A cross involving only one trait is
referred to as a monohybrid cross. Mendel crossed pure-breeding (also referred to as
true-breeding) smooth-seeded plants with a variety that had always produced wrinkled
seeds (60 fertilizations on 15 plants). All resulting seeds were smooth. The following
year, Mendel planted these seeds and allowed them to self-fertilize. He recovered 7324
seeds: 5474 smooth and 1850 wrinkled. To help with record keeping, generations were
labeled and numbered. The parental generation is denoted as the P1 generation. The
offspring of the P1 generation are the F1 generation (first filial). The self-fertilizing
F1 generation produced the F2 generation (second filial).
Inheritance of two alleles, S and s, in peas. Image from Purves et al., Life: The Science of
5 of 13
23/11/06 15:10
Intro to Genetics
Punnett square explaining the behavior of the S and s alleles. Image from Purves et al., Life:
The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman
P1: smooth X wrinkled
F1 : all smooth
F2 : 5474 smooth and 1850 wrinkled
Meiosis, a process unknown in Mendel's day, explains how the traits are inherited.
6 of 13
23/11/06 15:10
Intro to Genetics
The inheritance of the S and s alleles explained in light of meiosis. Image from Purves et
al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH
Freeman (www.whfreeman.com), used with permission.
Mendel studied seven traits which appeared in two discrete forms, rather than
continuous characters which are often difficult to distinguish. When "true-breeding"
tall plants were crossed with "true-breeding" short plants, all of the offspring were tall
plants. The parents in the cross were the P1 generation, and the offspring represented
the F1 generation. The trait referred to as tall was considered dominant, while short
was recessive. Dominant traits were defined by Mendel as those which appeared in the
F1 generation in crosses between true-breeding strains. Recessives were those which
"skipped" a generation, being expressed only when the dominant trait is absent.
Mendel's plants exhibited complete dominance, in which the phenotypic expression of
alleles was either dominant or recessive, not "in between".
When members of the F1 generation were crossed, Mendel recovered mostly tall
offspring, with some short ones also occurring. Upon statistically analyzing the F2
7 of 13
23/11/06 15:10
Intro to Genetics
generation, Mendel determined the ratio of tall to short plants was approximately 3:1.
Short plants have skipped the F1 generation, and show up in the F2 and succeeding
generations. Mendel concluded that the traits under study were governed by discrete
(separable) factors. The factors were inherited in pairs, with each generation having a
pair of trait factors. We now refer to these trait factors as alleles. Having traits
inherited in pairs allows for the observed phenomena of traits "skipping" generations.
Summary of Mendel's Results:
1. The F1 offspring showed only one of the two parental traits, and always the same
trait.
2. Results were always the same regardless of which parent donated the pollen (was
male).
3. The trait not shown in the F1 reappeared in the F2 in about 25% of the
offspring.
4. Traits remained unchanged when passed to offspring: they did not blend in any
offspring but behaved as separate units.
5. Reciprocal crosses showed each parent made an equal contribution to the
offspring.
Mendel's Conclusions:
1. Evidence indicated factors could be hidden or unexpressed, these are the
recessive traits.
2. The term phenotype refers to the outward appearance of a trait, while the term
genotype is used for the genetic makeup of an organism.
3. Male and female contributed equally to the offsprings' genetic makeup: therefore
the number of traits was probably two (the simplest solution).
4. Upper case letters are traditionally used to denote dominant traits, lower case
letters for recessives.
Mendel reasoned that factors must segregate from each other during gamete formation
(remember, meiosis was not yet known!) to retain the number of traits at 2. The
Principle of Segregation proposes the separation of paired factors during gamete
formation, with each gamete receiving one or the other factor, usually not both.
Organisms carry two alleles for every trait. These traits separate during the formation
of gametes.
A hypertext version (in German or English, annotated also available) of Mendel's
1865 paper is available by clicking here.
Dihybrid Crosses | Back to Top
When Mendel considered two traits per cross (dihybrid, as opposed to
single-trait-crosses, monohybrid), The resulting (F2) generation did not have 3:1
dominant:recessive phenotype ratios. The two traits, if considered to inherit
independently, fit into the principle of segregation. Instead of 4 possible genotypes
from a monohybrid cross, dihybrid crosses have as many as 16 possible genotypes.
8 of 13
23/11/06 15:10
Intro to Genetics
Mendel realized the need to conduct his experiments on more complex situations. He
performed experiments tracking two seed traits: shape and color. A cross concerning
two traits is known as a dihybrid cross.
Crosses With Two Traits
Smooth seeds (S) are dominant over wrinkled (s) seeds.
Yellow seed color (Y) is dominant over green (g).
Inheritance of two traits simultaneously, a dihybrid cross. The above graphic is from the
Genetics pages at McGill University (http://www.mcgill.ca/nrs/dihyb2.gif).
Again, meiosis helps us understand the behavior of alleles.
9 of 13
23/11/06 15:10
Intro to Genetics
The inheritance of two traits on different chromosomes can be explained by meiosis.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Methods, Results, and Conclusions
Mendel started with true-breeding plants that had smooth, yellow seeds and crossed
them with true-breeding plants having green, wrinkled seeds. All seeds in the F1 had
smooth yellow seeds. The F2 plants self-fertilized, and produced four phenotypes:
315 smooth yellow
108 smooth green
101 wrinkled yellow
32 wrinkled green
Mendel analyzed each trait for separate inheritance as if the other trait were not
10 of 13
23/11/06 15:10
Intro to Genetics
present.The 3:1 ratio was seen separately and was in accordance with the Principle of
Segregation. The segregation of S and s alleles must have happened independently of
the segregation of Y and y alleles. The chance of any gamete having a Y is 1/2; the
chance of any one gamete having a S is 1/2.The chance of a gamete having both Y and
S is the product of their individual chances (or 1/2 X 1/2 = 1/4). The chance of two
gametes forming any given genotype is 1/4 X 1/4 (remember, the product of their
individual chances). Thus, the Punnett Square has 16 boxes. Since there are more
possible combinations to produce a smooth yellow phenotype (SSYY, SsYy, SsYY,
and SSYy), that phenotype is more common in the F2.
From the results of the second experiment, Mendel formulated the Principle of
Independent Assortment -- that when gametes are formed, alleles assort independently.
If traits assort independent of each other during gamete formation, the results of the
dihybrid cross can make sense. Since Mendel's time, scientists have discovered
chromosomes and DNA. We now interpret the Principle of Independent Assortment as
alleles of genes on different chromosomes are inherited independently during the
formation of gametes. This was not known to Mendel.
Punnett squares deal only with probability of a genotype showing up in the next
generation. Usually if enough offspring are produced, Mendelian ratios will also be
produced.
Step 1 - definition of alleles and determination of dominance.
Step 2 - determination of alleles present in all different types of gametes.
Step 3 - construction of the square.
Step 4 - recombination of alleles into each small square.
Step 5 - Determination of Genotype and Phenotype ratios in the next generation.
Step 6 - Labeling of generations, for example P1, F1, etc.
While answering genetics problems, there are certain forms and protocols that will
make unintelligible problems easier to do. The term "true-breeding strain" is a code
word for homozygous. Dominant alleles are those that show up in the next generation
in crosses between two different "true-breeding strains". The key to any genetics
problem is the recessive phenotype (more properly the phenotype that represents the
recessive genotype). It is that organism whose genotype can be determined by
examination of the phenotype. Usually homozygous dominant and heterozygous
individuals have identical phenotypes (although their genotypes are different). This
becomes even more important in dihybrid crosses.
Mutations | Back to Top
Hugo de Vries, one of three turn-of-the-century scientists who rediscovered the work
of Mendel, recognized that occasional abrupt, sudden changes occurred in the patterns
11 of 13
23/11/06 15:10
Intro to Genetics
of inheritance in the primrose plant. These sudden changes he termed mutations. De
Vries proposed that new alleles arose by mutations. Charles Darwin, in his Origin of
Species, was unable to describe how heritable changes were passed on to subsequent
generations, or how new adaptations arose. Mutations provided answers to problems of
the appearance of novel adaptations. The patterns of Mendelian inheritance explained
the perseverance of rare traits in organisms, all of which increased variation, as you
recall that was a major facet of Darwin's theory.
Mendel's work was published in 1866 but not recognized until the early 1900s when
three scientists independently verified his principles, more than twenty years after his
death. Ignored by the scientific community during his lifetime, Mendel's work is now
a topic enjoyed by generations of biology students (;))
Genetic Terms | Back to Top
Definitions of terms. While we are discussing Mendel, we need to understand the
context of his times as well as how his work fits into the modern science of genetics.
Gene - a unit of inheritance that usually is directly responsible for one trait or
character.
Allele - an alternate form of a gene. Usually there are two alleles for every gene,
sometimes as many a three or four.
Homozygous - when the two alleles are the same.
Heterozygous - when the two alleles are different, in such cases the dominant allele is
expressed.
Dominant - a term applied to the trait (allele) that is expressed irregardless of the
second allele.
Recessive - a term applied to a trait that is only expressed when the second allele is the
same (e.g. short plants are homozygous for the recessive allele).
Phenotype - the physical expression of the allelic composition for the trait under
study.
Genotype - the allelic composition of an organism.
Punnett squares - probability diagram illustrating the possible offspring of a mating.
Links | Back to Top
History of Genetics Web Pages (University of California, Davis)
Quantitative Genetics Resources (University of Arizona)
MendelWeb Hey, how many folks have their own webpages over a century after their deaths?
Classic Papers in Genetics These files are downloadable in Adobe Acrobat format. The site has a
link to get the viewer from Adobe.
12 of 13
23/11/06 15:10
Intro to Genetics
Mendelian Genetics (Bio 181 at the University of Arizona) Lecture notes, a genetics tutorial, and
some very nice graphics.
Monohybrid Problem Set (The Biology Project, U of AZ) Tutorial on single-trait crosses.
Dihybrid Problem Set (The Biology Project, U of AZ) Tutorial on two-trait crosses.
The Virtual Fly Lab Conduct online genetics crosses with virtual Drosophila. An excellent, much
cited and visited site.
MIT Hypertextbook Chapter on Mendelain Genetics
The History of Genetics (Whitman College) An outline.
Course Outline in Genetics (McGill University) An outline and many quality graphics and
animations.
Interactive Pea Experiment (Bill Kendrick) Select peas to breed. Nice introduction to genetics
experiments.
Glossary of Genetics Terms
Text ©1992, 1994, 1997, 1999, 2000, 2001, by M.J. Farabee, all rights reserved. Use for educational
purposes is encouraged.
Back to Table of Contents | Go to GENE INTERACTIONS
Last modified:
13 of 13
23/11/06 15:10
Reading(s)
#7.2
Genetics/Mendelian Inheritence - Wikibooks, collection of open-cont... http://en.wikibooks.org/wiki/Genetics/Mendelian_Inheritence#Introdu...
Genetics/Mendelian Inheritence
< Genetics
Introduction
Gregor Johann Mendel was a monk in the Augustinian Monastery in the Brunn, Czech Republic. In 1854 he
began the experiments which started modern genetics. His work with garden peas, Pisum sativum, was vital
to our understanding of inheritance. He is known as the Father Of Genetics.
Mendel's Experiment
Mendel's first step was breeding pure breeding strains of peas. The traits he studied included:
Pea colour
Height
and whether the Peas were wrinkled or smooth.
Mendel crossed the pure breeding Parental Generation (designated P). He found that the first generation (F1)
was exclusively phenotypically one of the parental types. Mendel then crossed his F1 generation with itself.
He found that the F2 generation showed a surprising trait, three quarters were like the F1 generation, while
the remaining quarter were like the other Parents.
From this Mendel realised that there were two versions of each loci, one of which expressed dominance over
the other. He called this Biparticulate Inheritance. If a gene was following this 3:1 pattern it was said to be
segregating Normally.
By looking at multiple genes, Mendel showed that they were not linked to each other and that each loci he
studied had no influence over the others. He called this Independent assortment.
By studying cases where the Mendelian laws we can also learn a lot. For instance, if a gene isn't segragating
normally it may be sex linked. If two genes aren't Assorting Independantly they're probably on the same
chromosome
Mendel's laws are the first step to understanding Genetics, they lay down the basic concept of inheritance.
Retrieved from "http://en.wikibooks.org/wiki/Genetics/Mendelian_Inheritence"
Category: Genetics
This page was last modified 19:03, 2 July 2006.
details).
Privacy policy
About Wikibooks
Disclaimers
1 of 1
23/11/06 15:12
Reading(s) #8
CELL DIVISION: BINARY FISSION AND MITOSIS
Table of Contents
The Cell Cycle | Prokaryotic Cell Division | Eukaryotic Cell Division | Mitosis
Prophase | Metaphase | Anaphase | Telophase | Cytokinesis | Links
The Cell Cycle | Back to Top
Despite differences between prokaryotes and eukaryotes, there are several common
features in their cell division processes. Replication of the DNA must occur.
Segregation of the "original" and its "replica" follow. Cytokinesis ends the cell
division process. Whether the cell was eukaryotic or prokaryotic, these basic events
must occur.
Cytokinesis is the process where one cell splits off from its sister cell. It usually occurs
after cell division. The Cell Cycle is the sequence of growth, DNA replication, growth
and cell division that all cells go through. Beginning after cytokinesis, the daughter
cells are quite small and low on ATP. They acquire ATP and increase in size during
the G1 phase of Interphase. Most cells are observed in Interphase, the longest part of
the cell cycle. After acquiring sufficient size and ATP, the cells then undergo DNA
Synthesis (replication of the original DNA molecules, making identical copies, one
"new molecule" eventually destined for each new cell) which occurs during the S
phase. Since the formation of new DNA is an energy draining process, the cell
undergoes a second growth and energy acquisition stage, the G2 phase. The energy
acquired during G2 is used in cell division (in this case mitosis).
1 of 9
23/11/06 15:13
The cell cycle. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Regulation of the cell cycle is accomplished in several ways. Some cells divide rapidly
(beans, for example take 19 hours for the complete cycle; red blood cells must divide
at a rate of 2.5 million per second). Others, such as nerve cells, lose their capability to
divide once they reach maturity. Some cells, such as liver cells, retain but do not
normally utilize their capacity for division. Liver cells will divide if part of the liver is
removed. The division continues until the liver reaches its former size.
Cancer cells are those which undergo a series of rapid divisions such that the daughter
cells divide before they have reached "functional maturity". Environmental factors
such as changes in temperature and pH, and declining nutrient levels lead to declining
cell division rates. When cells stop dividing, they stop usually at a point late in the G1
phase, the R point (for restriction).
Prokaryotic Cell Division | Back to Top
Prokaryotes are much simpler in their organization than are eukaryotes. There are a
great many more organelles in eukaryotes, also more chromosomes. The usual method
of prokaryote cell division is termed binary fission. The prokaryotic chromosome is a
single DNA molecule that first replicates, then attaches each copy to a different part of
the cell membrane. When the cell begins to pull apart, the replicate and original
chromosomes are separated. Following cell splitting (cytokinesis), there are then two
cells of identical genetic composition (except for the rare chance of a spontaneous
mutation).
This animated GIF of binary fission is from:
http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap2.html#two_bact_groups
The prokaryote chromosome is much easier to manipulate than the eukaryotic one. We
thus know much more about the location of genes and their control in prokaryotes.
One consequence of this asexual method of reproduction is that all organisms in a
colony are genetic equals. When treating a bacterial disease, a drug that kills one
bacteria (of a specific type) will also kill all other members of that clone (colony) it
comes in contact with.
2 of 9
23/11/06 15:13
Rod-Shaped Bacterium, E. coli, dividing by binary fission (TEM x92,750). This image
is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
3 of 9
23/11/06 15:13
Rod-Shaped Bacterium, hemorrhagic E. coli, strain 0157:H7 (division) (SEM x22,810).
This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
Eukaryotic Cell Division | Back to Top
Due to their increased numbers of chromosomes, organelles and complexity, eukaryote
cell division is more complicated, although the same processes of replication,
segregation, and cytokinesis still occur.
Mitosis | Back to Top
Mitosis is the process of forming (generally) identical daughter cells by replicating and
dividing the original chromosomes, in effect making a cellular xerox. Commonly the
two processes of cell division are confused. Mitosis deals only with the segregation of
the chromosomes and organelles into daughter cells.
Click here to view an animated GIF of mitosis from
http://www.biology.uc.edu/vgenetic/mitosis/mitosis.htm.
Eukaryotic chromosomes occur in the cell in greater numbers than prokaryotic
chromosomes. The condensed replicated chromosomes have several points of interest.
The kinetochore is the point where microtubules of the spindle apparatus attach.
Replicated chromosomes consist of two molecules of DNA (along with their associated
histone proteins) known as chromatids. The area where both chromatids are in contact
with each other is known as the centromere the kinetochores are on the outer sides of
the centromere. Remember that chromosomes are condensed chromatin (DNA plus
histone proteins).
Structure of a eukaryotic chromosome. Image from Purves et al., Life: The Science of Biology,
During mitosis replicated chromosomes are positioned near the middle of the
4 of 9
23/11/06 15:13
cytoplasm and then segregated so that each daughter cell receives a copy of the
original DNA (if you start with 46 in the parent cell, you should end up with 46
chromosomes in each daughter cell). To do this cells utilize microtubules (referred to
as the spindle apparatus) to "pull" chromosomes into each "cell". The microtubules
have the 9+2 arrangement discussed earlier. Animal cells (except for a group of
worms known as nematodes) have a centriole. Plants and most other eukaryotic
organisms lack centrioles. Prokaryotes, of course, lack spindles and centrioles; the cell
membrane assumes this function when it pulls the by-then replicated chromosomes
apart during binary fission. Cells that contain centrioles also have a series of smaller
microtubules, the aster, that extend from the centrioles to the cell membrane. The aster
is thought to serve as a brace for the functioning of the spindle fibers.
Structure and main features of a spindle apparatus. Image from Purves et al., Life: The
The phases of mitosis are sometimes difficult to separate. Remember that the process
is a dynamic one, not the static process displayed of necessity in a textbook.
Prophase | Back to Top
Prophase is the first stage of mitosis proper. Chromatin condenses (remember that
chromatin/DNA replicate during Interphase), the nuclear envelope dissolves, centrioles
(if present) divide and migrate, kinetochores and kinetochore fibers form, and the
spindle forms.
5 of 9
23/11/06 15:13
Pea Plant Nuclear DNA, from Vicea faba (TEM x105,000). This image is copyright Dennis
6 of 9
23/11/06 15:13
The events of Prophase. Image from Purves et al., Life: The Science of Biology, 4th Edition, by
permission.
Metaphase | Back to Top
Metaphase follows Prophase. The chromosomes (which at this point consist of
chromatids held together by a centromere) migrate to the equator of the spindle, where
the spindles attach to the kinetochore fibers.
Anaphase | Back to Top
Anaphase begins with the separation of the centromeres, and the pulling of
chromosomes (we call them chromosomes after the centromeres are separated) to
opposite poles of the spindle.
7 of 9
23/11/06 15:13
The events of Metaphase and Anaphase. Image from Purves et al., Life: The Science of Biology,
Telophase | Back to Top
Telophase is when the chromosomes reach the poles of their respective spindles, the
nuclear envelope reforms, chromosomes uncoil into chromatin form, and the nucleolus
(which had disappeared during Prophase) reform. Where there was one cell there are
now two smaller cells each with exactly the same genetic information. These cells may
then develop into different adult forms via the processes of development.
The events of Telophase. Image from Purves et al., Life: The Science of Biology, 4th Edition, by
8 of 9
23/11/06 15:13
permission.
Cytokinesis | Back to Top
Cytokinesis is the process of splitting the daughter cells apart. Whereas mitosis is the
division of the nucleus, cytokinesis is the splitting of the cytoplasm and allocation of
the golgi, plastids and cytoplasm into each new cell.
Links | Back to Top
Access Excellence page on Mitosis
Cell Division and the Cell Cycle (University of Alberta): Similar to this page, but with its own
glossary and questions.
Amoeba Proteus Mitosis Small photomicrographs of protistan mitosis.
Cell Reproduction Notes from University of Georgia, plus some cool graphics of mitosis.
Phases of Mitosis U Texas QuickTime® movies of mitosis.
Animated Mitosis Yale University, a simplified series of cartoons about mitosis.
Mitosis U Southern Mississippi, Grayscale drawings and photomicrographs of mitosis stages.
Mitosis San Diego State U, shocked animation of the process. You will need the Shockwave
plugin to view. If you don't have it, you can download it from them.
McGill University Mitosis Page Quality site, with photos and downloadable animation and video.
Comparison of Mitosis and Meiosis Whitman College, table summarizing each process.
Whitefish Mitosis Review Cornell, photomicrographs of mitosis in whitefish. A nice review after
lab! Part of a more extensive page of Cell Division Tutorials.
Virtual Mitosis University of Cincinnati, Animated GIF and text about the stages of mitosis.
Text ©1992, 1994, 1997, 1998, 2000, 2001, by M.J. Farabee, all rights reserved. Use for educational
purposes is encouraged.
Back to Table of Contents | MEIOSIS AND SEXUAL REPRODUCTION
Last modified:
9 of 9
23/11/06 15:13
Reading(s) #9
Reading(s)
#9.1
CELL DIVISION: Meiosis...
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeios...
CELL DIVISION: MEIOSIS AND SEXUAL REPRODUCTION
Table of Contents
Meiosis | Ploidy | Life Cycles | Phases of Meiosis | Prophase I | Metaphase I
Anaphase I | Telophase I | Prophase II | Metaphase II | Anaphase II | Telophase II
Comparison of Mitosis and Meiosis | Gametogenesis | Links
Meiosis | Back to Top
Sexual reproduction occurs only in eukaryotes. During the formation of gametes, the
number of chromosomes is reduced by half, and returned to the full amount when the
two gametes fuse during fertilization.
Ploidy | Back to Top
Haploid and diploid are terms referring to the number of sets of chromosomes in a
cell. Gregor Mendel determined his peas had two sets of alleles, one from each parent.
Diploid organisms are those with two (di) sets. Human beings (except for their
gametes), most animals and many plants are diploid. We abbreviate diploid as 2n.
Ploidy is a term referring to the number of sets of chromosomes. Haploid
organisms/cells have only one set of chromosomes, abbreviated as n. Organisms with
more than two sets of chromosomes are termed polyploid. Chromosomes that carry the
same genes are termed homologous chromosomes. The alleles on homologous
chromosomes may differ, as in the case of heterozygous individuals. Organisms
(normally) receive one set of homologous chromosomes from each parent.
Meiosis is a special type of nuclear division which segregates one copy of each
homologous chromosome into each new "gamete". Mitosis maintains the cell's original
ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one
haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces
the number of sets of chromosomes by half, so that when gametic recombination
(fertilization) occurs the ploidy of the parents will be reestablished.
Most cells in the human body are produced by mitosis. These are the somatic (or
vegetative) line cells. Cells that become gametes are referred to as germ line cells. The
vast majority of cell divisions in the human body are mitotic, with meiosis being
restricted to the gonads.
Life Cycles | Back to Top
Life cycles are a diagrammatic representation of the events in the organism's
development and reproduction. When interpreting life cycles, pay close attention to
the ploidy level of particular parts of the cycle and where in the life cycle meiosis
1 of 14
31/03/07 05:54
occurs. For example, animal life cycles have a dominant diploid phase, with the
gametic (haploid) phase being a relative few cells. Most of the cells in your body are
diploid, germ line diploid cells will undergo meiosis to produce gametes, with
fertilization closely following meiosis.
Plant life cycles have two sequential phases that are termed alternation of generations.
The sporophyte phase is "diploid", and is that part of the life cycle in which meiosis
occurs. However, many plant species are thought to arise by polyploidy, and the use of
"diploid" in the last sentence was meant to indicate that the greater number of
chromosome sets occur in this phase. The gametophyte phase is "haploid", and is the
part of the life cycle in which gametes are produced (by mitosis of haploid cells). In
flowering plants (angiosperms) the multicelled visible plant (leaf, stem, etc.) is
sporophyte, while pollen and ovaries contain the male and female gametophytes,
respectively. Plant life cycles differ from animal ones by adding a phase (the haploid
gametophyte) after meiosis and before the production of gametes.
Many protists and fungi have a haploid dominated life cycle. The dominant phase is
haploid, while the diploid phase is only a few cells (often only the single celled
zygote, as in Chlamydomonas ). Many protists reproduce by mitosis until their
environment deteriorates, then they undergo sexual reproduction to produce a resting
zygotic cyst.
Phases of Meiosis | Back to Top
Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II
(Division). Meiosis produces 4 haploid cells. Mitosis produces 2 diploid cells. The old
name for meiosis was reduction/ division. Meiosis I reduces the ploidy level from 2n
to n (reduction) while Meiosis II divides the remaining set of chromosomes in a
mitosis-like process (division). Most of the differences between the processes occur
during Meiosis I.
The above image is from http://www.biology.uc.edu/vgenetic/meiosis/
Prophase I | Back to Top
Prophase I has a unique event -- the pairing (by an as yet undiscovered mechanism) of
homologous chromosomes. Synapsis is the process of linking of the replicated
homologous chromosomes. The resulting chromosome is termed a tetrad, being
composed of two chromatids from each chromosome, forming a thick (4-strand)
structure. Crossing-over may occur at this point. During crossing-over chromatids
break and may be reattached to a different homologous chromosome.
2 of 14
31/03/07 05:54
The alleles on this tetrad:
ABCDEFG
ABCDEFG
abcdefg
abcdefg
will produce the following chromosomes if there is a crossing-over event between the
2nd and 3rd chromosomes from the top:
ABCDEFG
ABcdefg
abCDEFG
abcdefg
Thus, instead of producing only two types of chromosome (all capital or all lower
case), four different chromosomes are produced. This doubles the variability of
gamete genotypes. The occurrence of a crossing-over is indicated by a special
structure, a chiasma (plural chiasmata) since the recombined inner alleles will align
more with others of the same type (e.g. a with a, B with B). Near the end of Prophase
I, the homologous chromosomes begin to separate slightly, although they remain
attached at chiasmata.
3 of 14
31/03/07 05:54
Crossing-over between homologous chromosomes produces chromosomes with new
associations of genes and alleles. Image from Purves et al., Life: The Science of Biology, 4th
with permission.
Events of Prophase I (save for synapsis and crossing over) are similar to those in
Prophase of mitosis: chromatin condenses into chromosomes, the nucleolus dissolves,
nuclear membrane is disassembled, and the spindle apparatus forms.
4 of 14
31/03/07 05:54
Major events in Prophase I. Image from Purves et al., Life: The Science of Biology, 4th Edition,
permission.
Metaphase I | Back to Top
Metaphase I is when tetrads line-up along the equator of the spindle. Spindle fibers
attach to the centromere region of each homologous chromosome pair. Other
metaphase events as in mitosis.
Anaphase I | Back to Top
Anaphase I is when the tetrads separate, and are drawn to opposite poles by the spindle
fibers. The centromeres in Anaphase I remain intact.
Events in prophase and metaphse I. Image from Purves et al., Life: The Science of Biology, 4th
5 of 14
31/03/07 05:54
with permission.
Telophase I | Back to Top
Telophase I is similar to Telophase of mitosis, except that only one set of (replicated)
chromosomes is in each "cell". Depending on species, new nuclear envelopes may or
may not form. Some animal cells may have division of the centrioles during this
phase.
The events of Telophase I. Image from Purves et al., Life: The Science of Biology, 4th Edition, by
permission.
Prophase II | Back to Top
During Prophase II, nuclear envelopes (if they formed during Telophase I) dissolve,
and spindle fibers reform. All else is as in Prophase of mitosis. Indeed Meiosis II is
very similar to mitosis.
6 of 14
31/03/07 05:54
The events of Prophase II. Image from Purves et al., Life: The Science of Biology, 4th Edition, by
permission.
Metaphase II | Back to Top
Metaphase II is similar to mitosis, with spindles moving chromosomes into equatorial
area and attaching to the opposite sides of the centromeres in the kinetochore region.
Anaphase II | Back to Top
During Anaphase II, the centromeres split and the former chromatids (now
chromosomes) are segregated into opposite sides of the cell.
7 of 14
31/03/07 05:54
The events of Metaphase II and Anaphase II. Image from Purves et al., Life: The Science of
Telophase II | Back to Top
Telophase II is identical to Telophase of mitosis. Cytokinesis separates the cells.
The events of Telophase II. Image from Purves et al., Life: The Science of Biology, 4th Edition,
permission.
Comparison of Mitosis and Meiosis | Back to Top
Mitosis maintains ploidy level, while meiosis reduces it. Meiosis may be considered a
reduction phase followed by a slightly altered mitosis. Meiosis occurs in a relative few
cells of a multicellular organism, while mitosis is more common.
8 of 14
31/03/07 05:54
9 of 14
31/03/07 05:54
10 of 14
31/03/07 05:54
Comparison of the events in Mitosis and Meiosis. Images from Purves et al., Life: The Science
of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman
Gametogenesis | Back to Top
Gametogenesis is the process of forming gametes (by definition haploid, n) from
diploid cells of the germ line. Spermatogenesis is the process of forming sperm cells
by meiosis (in animals, by mitosis in plants) in specialized organs known as gonads (in
males these are termed testes). After division the cells undergo differentiation to
become sperm cells. Oogenesis is the process of forming an ovum (egg) by meiosis (in
animals, by mitosis in the gametophyte in plants) in specialized gonads known as
ovaries. Whereas in spermatogenesis all 4 meiotic products develop into gametes,
oogenesis places most of the cytoplasm into the large egg. The other cells, the polar
bodies, do not develop. This all the cytoplasm and organelles go into the egg. Human
males produce 200,000,000 sperm per day, while the female produces one egg
(usually) each menstrual cycle.
11 of 14
31/03/07 05:54
Gametogenesis. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Spermatogenesis
Sperm production begins at puberty at continues throughout life, with several hundred
million sperm being produced each day. Once sperm form they move into the
epididymis, where they mature and are stored.
12 of 14
31/03/07 05:54
Human Sperm (SEM x5,785). This image is copyright Dennis Kunkel at
Oogenesis
The ovary contains many follicles composed of a developing egg surrounded by an
outer layer of follicle cells. Each egg begins oogenesis as a primary oocyte. At birth
each female carries a lifetime supply of developing oocytes, each of which is in
Prophase I. A developing egg (secondary oocyte) is released each month from puberty
until menopause, a total of 400-500 eggs.
13 of 14
31/03/07 05:54
Oogenesis. The above image is from http://www.grad.ttuhsc.edu/courses/histo/notes/female.html.
Links | Back to Top
Access Excellence page on Mitosis
Cell Division and the Cell Cycle (University of Alberta): Similar to this page, but with its own
glossary and questions.
Amoeba Proteus Mitosis Small photomicrographs of protistan mitosis.
Animated Meiosis Yale University, a simplified series of cartoons about meiosis.
Meiosis Tutorial North Carolina State University, animations and 3-D graphics.
McGill University Mitosis Page Quality site, with photos and downloadable animation and video.
Virtual Meiosis University of Cincinnati, Animated GIF and text/images to explain meiosis.
Text ©1992, 1994, 1997, 1998, 2000, 2001, 2007, by M.J. Farabee, all rights reserved. Use for
educational purposes is encouraged.
Back to Table of Contents | Mitosis Page
Last modified:
14 of 14
31/03/07 05:54
Reading(s)
#9.2
Meiosis - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Meiosis
Meiosis
In biology, meiosis is the process that allows one diploid
cell to divide in a special way to generate haploid cells in
eukaryotes. The word "meiosis" comes from the Greek
meioun, meaning "to make smaller," since it results in a
reduction in chromosome number.
Meiosis is essential for sexual reproduction. It therefore
occurs in most eukaryotes, including single-celled
organisms. A few eukaryotes, notably the Bdelloid
rotifers, have lost the ability to carry out meiosis and
acquired the ability to reproduce by parthenogenesis.
Meiosis does not occur in archaea or prokaryotes, which
reproduce by asexual cell division processes.
During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA called
chromosomes, undergoes DNA replication followed by two rounds of division, resulting in haploid cells
called gametes. Each gamete contains one complete set of chromosomes, or half of the genetic content of the
original cell. These resultant haploid cells can fuse with other haploid cells of the opposite gender or mating
type during fertilization to create a new diploid cell, or zygote. Thus, the division mechanism of meiosis is a
reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of
each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a
unique genetic blueprint encoded in its DNA. In other words, meiosis is the process that produces genetic
variation.
Biochemically, meiosis uses some of the same mechanisms employed during mitosis to accomplish the
redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing
and recombination between homologous chromosomes, which enable them to separate from each other.
Contents
1 History
2 Occurrence of meiosis in eukaryotic life cycles
3 Process
3.1 Meiosis I
3.1.1 Prophase I
3.1.2 Metaphase I
3.1.3 Anaphase I
3.1.4 Telophase I
4 =Meiosis II
5 Significance of meiosis
6 Nondisjunction
7 Meiosis in humans
8 External links
1 of 6
23/11/06 15:14
9 See also
History
Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by noted German
biologist Oscar Hertwig (1849-1922). It was described again in 1883, at the level of chromosomes, by
Belgian zoologist Edouard Van Beneden (1846-1910), in Ascaris worms' eggs. The significance of meiosis
for reproduction and inheritance, however, was described only in 1890 by German biologist August
Weismann (1834-1914), who noted that two cell divisions were necessary to transform one diploid cell into
four haploid cells if the number of chromosomes had to be maintained. In 1911 the American geneticist
Thomas Hunt Morgan (1866-1945) observed crossover in Drosophila melanogaster meiosis and provided
the first true genetic interpretation of meiosis.
Occurrence of meiosis in eukaryotic life cycles
Meiosis occurs in all eukaryotic life cycles involving sexual reproduction, comprising
of the constant cyclical process of meiosis and fertilization. This takes place alongside
normal mitotic cell division. In multicellular organisms, there is an intermediary step
between the diploid and haploid transition where the organism grows. The organism
will then produce the germ cells that continue in the life cycle. The rest of the cells,
called somatic cells, function within the organism and will die with it.
The organism phase of the life cycle can occur between the haploid to diploid
transition or the diploid to haploid transition. Some species are diploid, grown from a
diploid cell called the zygote. Others are haploid instead, spawned by the proliferation
and differentiation of a single haploid cell called the gamete. Humans, for example, are
diploid creatures. Human stem cells undergo meiosis to create haploid gametes, which
are sperm cells for males or ova for females. These gametes then fertilize in the uterus
of the female, producing a diploid zygote. The zygote undergoes progressive stages of
mitosis and differentiation to create an embryo, the early stage of human life.
Gametic life cycle.
Zygotic life cycle.
There are three types of life cycles that utilise sexual reproduction, differentiated by
the location of the organisms stage.
In the gametic life cycle, of which humans are a part, the living organism is diploid in
nature. Here, we will generalize the example of human reproduction stated previously.
Sporic life cycle.
The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes,
which fertilize to form the zygote. The diploid zygote undergoes repeated cellular
division by mitosis to grow into the organism. Mitosis is a related process to meiosis that creates two cells that
are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and
meiosis creates germ cells.
In the zygotic life cycle, the living organism is haploid. Two organisms of opposing gender contribute their
haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four
haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa are
members of the zygotic life cycle.
2 of 6
23/11/06 15:14
Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states.
Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line
cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid
organism. The haploid organism's germ cells then combine with another haploid organism's cells, creating
the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again.
The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles, and indeed its
diagram supports this conclusion.
Process
Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. However,
the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic
cell cycle.
Interphase is divided into three phases:
Gap 1 (G1 ) phase: Characterized by increase in cell size due to accelerated manufacture of organelles,
proteins, and other cellular matter.
Synthesis (S) phase: The genetic material is replicated: each of its single stranded components replicates
into a double stranded structure. The cell thereby transforms from diploid to tetraploid.
Gap 2 (G2 ) phase: The cell continues to grow.
Interphase is immediately followed by meiosis I and meiosis II. Meiosis I consists of segregating the
homologous chromosomes from each other, then dividing the tetraploid cell into two diploid cells each
containing one of the segregates. Meiosis II consists of decoupling each chromosome's sister strands
(chromatids), segregating the DNA into two sets of strands (each set containing one of each homolog), and
dividing both diploid cells to produce four haploid cells. Meiosis I and II are both divided into prophase,
metaphase, anaphase, and telophase subphases, similar in purpose to their analogous subphases in the mitotic
cell cycle. Therefore, meiosis encompasses the interphase (G1 , S, G2 ), meiosis I (prophase I, metaphase I,
anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
Meiosis I
Prophase I
In the prophase stage, the cell's genetic material, which is normally in a loosely arranged pile known as
chromatin, condenses into visible threadlike structures. Along the thread, centromeres are visible as small
beads of tightly coiled chromatin.
The first stage of Prophase I is the leptotene stage, during which individual chromosomes begin to condense
into long strands within the nucleus.
The zygotene stage then occurs as the chromosomes approximately line up with each other into homologous
chromosomes. The combined homologous chromosomes are said to be bivalent. They may also be referred
to as a tetrad, a reference to the four sister chromatids. During this stage, one percent of DNA that wasn't
replicated during S phase is replicated. The significance of this cleanup act is unclear.
The pachytene stage heralds crossing over. Nonsister chromatids of homologous chromosomes randomly
exchange segments of genetic information. Because the chromosomes cannot be distinguished in the
synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
3 of 6
23/11/06 15:14
During the diplotene stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and
begin to repel each other. The chromosomes themselves uncoil a bit, allowing some transcription of DNA.
They are held together by virtue of recombination nodules, betraying the sites of previous crossing over, the
chiasmata.
Chromosomes recondense during the diakinesis stage. Sites of crossing over entangle together, effectively
overlapping, making chiasmata clearly visible. In general, every chromosome will have crossed over at least
once. This means that the pairs of homologous chromosomes are so tightly packed they exchange genetic
material. The nucleoli disappears and the nuclear membrane disintegrates into vesicles.
During these stages, centrioles are migrating to the two poles of the cell. These centrioles, which were
duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout microtubules,
essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it
disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor,
pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a
track. There are two kinetochores on each tetrad, one for each centrosome. Prophase I is the longest phase in
meiosis.
Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will
interact with microtubules from the opposite centriole. These are called nonkinetochore microtubules.
Metaphase I
As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous
chromosomes align equidistant above and below an imaginary equatorial plane, due to continuous
counterbalancing forces exerted by the two kinetochores of the bivalent. Because of independent assortment,
the orientation of the bivalent along the plane is random. Maternal or paternal homologues may point to
either pole.
Anaphase I
Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous
chromosomes apart. Since each chromosome only has one kinetochore, whole chromosomes are pulled
toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids.
Nonkinetochore microtubules lengthen, pushing the centrioles further apart. The cell elongates in
preparation for division down the middle. Also Homologous pairs line up together at the metaphase plate.
Telophase I
The first meiotic division effectively ends when the centromeres arrive at the poles. Each daughter cell now
has half the number of chromosomes but each chromosome consists of a pair of chromatids. The
microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each
haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane
in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two
daughter cells.
Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this
stage. Note that many plants skip telophase I and interphase II, going immediately into prophase II.
=Meiosis II
4 of 6
23/11/06 15:14
Prophase II takes an inversely proportional time compared to telophase I. In this prophase we see the
disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the
chromatids. Centrioles move to the polar regions and are arranged by spindle fibres. The new equatorial
plane is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plane.
In metaphase II, the centromeres contain three kinetochores, organizing fibers from the centrosomes on
each side.
This is followed by anaphase II, where the centromeres are cleaved, allowing the kinetochores to pull the
sister chromatids apart. The sister chromatids by convention are now called sister chromosomes, and they are
pulled toward opposing poles.
The process ends with telophase II, which is similar to telophase I, marked by uncoiling, lengthening, and
disappearance of the chromosomes occur as the disappearance of the microtubules. Nuclear envelopes
reform; cleavage or cell wall formation eventually produces a total of four daughter cells, each with a
haploid set of chromosomes. Meiosis is complete.
Significance of meiosis
Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count,
fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the
previous generation. Successive generations would have an exponential increase in chromosome count,
resulting in an unwieldy genome that would cripple the reproductive fitness of the species. Polyploidy, the
state of having three or more sets of chromosomes, also results in developmental abnormalities or lethality.
Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring.
Recombination and independent assortment allow for a greater diversity of genotypes in the population. As a
system of creating diversity, meiosis allows a species to maintain stability under environmental changes.
Nondisjunction
The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction.
When the separation is not normal, it is called nondisjunction. This results in the production of gametes
which have either more or less of the usual amount of genetic material, and is a common mechanism for
trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II phases of cellular
reproduction, or during mitosis.
This is a cause of several medical conditions in humans, including:
Down Syndrome - trisomy of chromosome 21
Patau Syndrome - trisomy of chromosome 13
Edward Syndrome - trisomy of chromosome 18
Klinefelter Syndrome - an extra X chromosome in males
Turner Syndrome - only one X chromosome present in females
XYY Syndrome - an extra Y chromosome in males
Triple X Syndrome - an extra X chromosome in females
Meiosis in humans
In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem
cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called
5 of 6
23/11/06 15:14
the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small
number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until
fertilization. The process of meiosis in females is called oogenesis, and differs from the typical meiosis in that
it features a long period of meiotic arrest known as the Dictyate stage and lacks the assistance of
centrosomes.
In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm.
These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced
at a steady pace. The process of meiosis in males is called spermatogenesis.
External links
Comparison of Meiosis and Mitosis (1)
(http://www.maxanim.com/genetics/Comprarison%20of%20Meiosis%20and%20Mitosis/Comprarison%20of%20Mei
Stages of Meiosis (2)
(http://www.maxanim.com/genetics/Stages%20of%20Meiosis/Stages%20of%20Meiosis.htm) (Flash
Animations)
See also
Mitosis
Ploidy
Spermatogenesis
Oogenesis
Retrieved from "http://en.wikipedia.org/wiki/Meiosis"
Category: Cell biology
6 of 6
23/11/06 15:14
Reading(s) #10
Reading(s)
#10.1
Terminator (genetics) - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Terminator_(genetics)
Terminator (genetics)
In genetics, a terminator, or transcription terminator is a section of genetic sequence that marks the end of
gene or operon on genomic DNA for transcription.
In prokaryotes, two classes of transcription terminators are known:
1. Intrinsic transcription terminators where a hairpin structure forms within the nascent transcript that
disrupts the mRNA-DNA-RNA polymerase ternary complex.
2. Rho-dependent transcription terminators that require Rho factor, an RNA helicase protein complex to
disrupt the nascent mRNA-DNA-RNA polymerase ternary complex.
In eukaryotes, terminators are recognized by protein factors that co-transcriptionally cleave the nascent RNA
at a polyadenylation signal, halting further elongation of the transcript by RNA polymerase. The subsequent
addition of the poly-A tail at this site stabilizes the mRNA and allows it to be exported outside the nucleus.
Terminator sequences are distinct from termination codons that occur in the mRNA and are the stopping
signal for translation, which may also be called nonsense codons.
A transcription terminator must also be distinguished from the dideoxynucleotides added to a dye
terminator sequencing.
Retrieved from "http://en.wikipedia.org/wiki/Terminator_%28genetics%29"
Categories: Genetics stubs | Genetics
details.)
1 of 1
23/11/06 15:14
Reading(s)
#10.2
Terminator Technology - Wikipedia, the free encyclopedia
Terminator Technology
Terminator Technology is the colloquial name given to proposed methods for restricting the use of
genetically modified plants by causing second generation seeds to be sterile. The technology was under
development by the U.S. Department of Agriculture and Delta and Pine Land Company in the 1990s and is
not yet commercially available. Because some stakeholders expressed concerns that this technology might
lead to dependence for poor smallholder farmers, Monsanto, an agricultural products company and the
world's biggest seed supplier, pledged
(http://www.guardian.co.uk/international/story/0,3604,260202,00.html) not to commercialize the
technology even if and when it becomes commercially available.
The technology was discussed during the 8th Conference of the Parties to the UN's Convention on Biological
Diversity in Curitiba, Brazil, March 20-31, 2006.
Terminator Technology is one form of Genetic Use Restriction Technologies (GURT). There are
conceptually two types of GURT.
1. V-GURT This type of GURT produces sterile seeds meaning that a farmer that had purchased seeds
containing v-GURT technology could not save the seed from this crop for future planting. This would not
have an immediate impact on the large number of farmers who use hybrid seeds, as they do not produce
their own planting seeds, and instead buy specialized hybrid seeds from seed production companies. The
technology is restricted at the plant variety level - hence the term V-GURT. Manufacturers of genetically
enhanced crops would use this technology to protect their products from unauthorised use.
2. T-GURT. A second type of GURT modifies a crop in such a way that the genetic enhancement
engineered into the crop does not function until the crop plant is treated with a chemical that is sold by the
biotechnology company. Farmers can save seeds for use each year. However, they do not get to use the
enhanced trait in the crop unless they purchase the activator compound. The technology is restricted at the
trait level - hence the term T-GURT.
Possible Advantages of GURT technology
1. An Incentive to the Development of New Plant Varieties
Where effective intellectual property protection systems don't exist or are not enforced, GURTs could be an
alternative to stimulate plant developing activities by biotech firms.
2. Improved Farm Management
Non-viable seed produced on V-GURT plants will reduce the propagation of volunteer plants. Volunteer
plants can become an economic problem for larger-scale mechanized farming systems that incorporate crop
rotation.
3. Improved grain quality
Under warm, wet harvest conditions non V-GURT grain can sprout, which lowers the quality of grain
produced. It is speculated that this problem would not occur with the use of V-GURT grain varieties.
1 of 2
23/11/06 15:15
Terminator Technology - Wikipedia, the free encyclopedia
4. Biosafety
Use of V-GURT technology could prevent escape of transgenes into wild relatives and prevent any impact
on biodiversity. Crops modified to produce non-food products could be armed with GURT technology to
prevent accidental transmission of these traits into crops destined for foods.
Possible Disadvantages of GURT Technology
1. Transmission of the "Terminator" trait to wild plants, or cultivated plants whose seeds are saved
There is a concern that V-GURT plants could cross-pollinate with non-genetically modified plants, either in
the wild or on the fields of farmers who do not adopt the technology. Though the V-GURT plants are
supposed to produce sterile seeds, there is concern that this trait will not be expressed in the first generation
of a small percentage of these plants, but be expressed in later generations. This does not seem to be much of
a problem in the wild, as a sterile plant would naturally be selected out of a population within one generation
of trait expression. This is however a problem in some farming systems, especially for indigenous groups
who save seed rather than purchase it from developers. The loss of the ability for such farmers to save seed
may lead to decreased agroecological biodiversity on their farms and decreased yields of affected crops.
2. Safety of Food produced from GURT crops
As with all Genetically Modified crops the food safety of GURT technology would need to be assessed when
and if a commercial release of a GURT containing crop was proposed.
3. The inequitable distribution of means; the targeting of vulnerable classes
In addition to potential biological and ecological harms, there is both an economic and normative concern
that small farmers, indigenous peoples, and entire rural communities could be made dependent on
agro-industry corporations for seed.
4. A presumption of entitlement
As with many other technologies, there is debate as to the role and responsibility implicit in the normative
assumptions involved in producing GURTs. The issue is distinct from the conflict surrounding the
production of GM foods generally, in that GURT products are specifically designed to affect future
generations and potentially have a distinct impact on human and ecological health and livelihoods. Thus,
some believe that in making decisions regarding such products, considerations should extend beyond what is
legally permissible.
Retrieved from "http://en.wikipedia.org/wiki/Terminator_Technology"
details.)
2 of 2
23/11/06 15:15
Reading(s)
#10.3
Genetically modified organism - Wikipedia, the free encyclopedia
Genetically modified organism
A genetically modified organism (GMO) is an organism whose genetic material
has been altered using techniques in genetics generally known as recombinant
DNA technology. Recombinant DNA technology is the ability to combine DNA
molecules from different sources into one molecule in a test tube. Thus, the
abilities or the phenotype of the organism, or the proteins it produces, can be
altered through the modification of its genes.
The term generally does not cover organisms whose genetic makeup has been
altered by conventional cross breeding or by "mutagenesis" breeding, as these
methods predate the discovery of the recombinant DNA techniques. Technically
speaking, however, such techniques are by definition genetic modification.
A tobacco plant which has
Examples of GMOs are diverse, and include transgenic experimental animals
been genetically
such as mice, several fish species, transgenic plants, or various microscopic
engineered to express a
organisms altered for the purposes of genetic research or for the production of
gene taken from fireflies.
pharmaceuticals. The term "genetically modified organism" does not necessarily
imply, but does include, transgenic substitution of genes from another species,
and research is actively being conducted in this field. For example, genes for fluorescent proteins can be
co-expressed with complex proteins in cultured cells to facilitate study by biologists, and modified organisms
are used in researching the mechanisms of cancer and other diseases.
Contents
1
2
3
4
5
6
7
8
9
History of GMO
Methods of genetic modification
Genetic modification of plants
Genetic modification of animals
Controversy
Transgenics featured in fiction
References
See also
External links
History of GMO
The first GMO was created in 1973 by Stanley N. Cohen and Herbert Boyer, demonstrating the creation of a
functional organism that combined and replicated genetic information from different species. [1]
(http://www.genomenewsnetwork.org/resources/timeline/1973_Boyer.php) . In mid-1974, very soon after
the first GMO was created, scientists called for and observed a voluntary moratorium on certain recombinant
DNA experiments. One goal of the moratorium was to provide time for a conference that would evaluate the
state of the new technology and the risks, if any, associated with it. That conference concluded that
recombinant DNA research should proceed but under strict guidelines. Such guidelines were subsequently
promulgated by the National Institutes of Health in the United States and by comparable bodies in other
1 of 6
23/11/06 15:15
countries. These guidelines form the basis upon which GMOs are regulated to this day. [2]
(http://nobelprize.org/chemistry/articles/berg/)
The first transgenic animals were mice created by Rudolf Jaenisch in 1974. Jaenish successfully managed to
insert foreign DNA into the early-stage mouse embryos; the resulting mice carried the modified gene in all
their tissues. Subsequent experiments, injecting leukemia genes to early mouse embryos using a retrovirus
vector, proved the genes integrated not only to the mice themselves, but also to their progeny.
Methods of genetic modification
Genetic modification involves genetic engineering, also known as gene splicing,
a technique to splice together DNA fragments from more than one organism and
thus preparing a "recombinant" DNA molecule in a test tube, producing a single
piece of genetic material containing the original information from multiple
fragments which can then be inserted into another organism. This is achieved by
cutting up DNA molecules with restriction enzymes and splicing these fragments
together using DNA ligase. A transgenic organism that contains such DNA
sequences from a foreign organism integrated into its own genome, the term
"transgenic" literally means across gene. A mouse or fish engineered to express
the green fluorescence protein, for example, would be considered a transgenic
organism, since the gene coding for the protein originated from a species of
jellyfish.
With current technology, transgenic organisms can be produced with only a very
small proportion of extraneous DNA. For example, the genome of most
mammals contains three billion basepairs of DNA, while it becomes relatively
difficult to insert more than 10,000 to 20,000 basepairs of foreign DNA. More
sophisticated techniques using yeast artificial chromosomes and bacterial
artificial chromosomes allow insertions of up to 320,000 basepairs [3]
The double-helix structure
of DNA allows
modification of plasmids to
take place.
(http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11884478&quer
- approximately 0.01% of the total genome. In concept, multiple rounds of transgenesis or interbreeding of
transgenics could lead to organisms with a higher proportion of foreign DNA, but cost and time
considerations prevent this.
In order to introduce new DNA into the receiving host, vectors are used. Vectors range from small circular
pieces of DNA such as plasmids, to various viruses that can carry and transmit genetic information. Three
processes are known by which the genetic composition of bacteria can be altered.
Transformation is a process by which some bacteria are naturally capable of taking up DNA to acquire new
genetic traits. This phenomenon was discovered in cows by Frederick Griffith in 1928, although the fact that
it was specifically DNA molecules that carried the genetic information was not proven until 1944. Bacteria
that are competent to undergo transformation are frequently used in molecular biology. The foreign DNA
uptake is facilitated by the presence of certain cations, such as Ca2+, or by the use of electric current
(electroporation). Transformation does not normally integrate new DNA into the bacterial chromosome.
Instead, it remains on a plasmid.
In conjugation, DNA is transferred from one bacterium to another via a temporary connecting tube of
protein called a pilus (a process analogous to but biologically distinct from mating). A plasmid is transferred
through the pilus. Conjugation is not widely used for the artificial genetic modification of bacteria, but
2 of 6
23/11/06 15:15
happens often in nature.
Transduction refers to the introduction of new DNA into a bacterial cell by a bacteriophage, a virus that
infects bacteria.
In order to gain knowledge about a particular gene's function, researchers often use knock out organisms.
These organisms have a specific gene that has been functionally destroyed or "knocked out." They are used
extensively in disease research with model organisms. For example, when investigating the cause of cystic
fibrosis, researchers identified the CFTR gene as a likely candidate for the disease, found the mouse
equivalent, bred a mouse with this gene "knocked out", and noted that the knockout mouse also had cystic
fibrosis.
Genetic modification of plants
Genetic modification of animals
Like bacteria and plants, animals can be genetically modified by viral infection. However, the genetic
modification occurs only in those cells that become infected, and in most cases these cells are eventually
eliminated by the immune system. In some cases it is possible to use the gene-transferring ability of viruses
for gene therapy, i.e. to correct diseases caused by a defective gene by supplying a normal copy of the gene.
Permanent genetic modification of entire animals can be accomplished in mice. The process begins by first
genetically modifying a mouse embryonic stem cell. This is normally done by physically introducing into
the cell a plasmid that can integrate into the genome by a process known as transfection [4]
(http://www.yotor.com/wiki/en/tr/Transfection.htm) . During transfection the DNA integrates into the animal
genome via non-homologous recombination. This altered cell is implanted into a blastocyst (an early
embryo), which is then implanted into the uterus of a female mouse. A pup born from this blastocyst will be
a chimera containing some cells derived from the unmodified cells of the blastocyst and some derived from
the modified stem cell. By selecting mice whose germ cells (sperm- or egg-producing cells) developed from
the modified cell and interbreeding them, pups that contain the genetic modification in all of their cells will
be born. Baylor College of Medicine currently has one of the largest transgenic mice facilities in the country.
There has also been the genetically manipulated bull Herman with 55 offspring. A human gene was built
into his genetic code while in an early embryonic stage in 1990. As a result, milk from his female
descendants contained the human protein lactoferrine, which can be used as medicine, but it was present at
such low levels that it was not profitable to extract them.
Insects can be genetically modified by injecting them with artificial transposons and a source of the enzyme
transposase. The transposon, which can include new genes, is then integrated into the genome. Such
insertions are unstable and can 'jump-out' in the presence of transposase.
Transgenic fish are often created by microinjection. First generation is mosaic but several lines have been
produced with the transgene incorporated into the germ line and transgenic fish can then be produced
"naturally" by crossing male and female gametes. Although many types of transgenic fish exist (e.g. for
increased cold tolerance, antibiotic production, ornamental Glofish etc), the main focus had been on
so-called growth hormone transgenic fish, mainly salmonids, tilapias and carps. These fish have an
over-production of growth hormone which results in increased growth rate from a few percent up to 30-40
times that of wild-types. In some species, final size is increased as well as growth rate providing an incentive
for commercial breeders to farm such fish. However, ecological concerns over potential negative effects of
transgenic fish in nature largely prevent the commencement of commercial production. A large and
important portion of the research on transgenic fish today is therefore focused on environmental
3 of 6
23/11/06 15:15
risk-assessment of GH-transgenic fish. Problems encountered and advances within this field are summarized
in Devlin et al (2006).
Controversy
See also Genetically modified food and Transgenic plants
Genetic modification (GM) is not the subject of controversy in its own right [5]
(http://www.csa.com/hottopics/gmfood/overview.php) . Some see the science itself as intolerable meddling
with "natural" order, despite many known examples of natural genetic crossings occurring throughout
history (see for example horizontal gene transfer). While some would like to see it banned, others push
simply for required labeling of genetically modified food. Other controversies include the definition of
patent and property pertaining to products of genetic engineering and the possibility of unforeseen global
side effects as a result of modified organisms proliferating. The basic ethical issues involved in genetic
research are discussed in the article on genetic engineering.
In 1986, field tests of a bacterium genetically engineered to protect plants from frost damage at a small
biotechnology company called Advanced Genetic Sciences of Oakland, California, were repeatedly delayed
by opponents of biotechnology. Also in 1986, a proposed field test of a microbe genetically engineered for
a pest resistance protein by Monsanto was dropped.[6]
(http://homepages.uel.ac.uk/J.Mottley/Newchapt.html)
In 2004, Mendocino County, California became the first county in the United States to ban the production of
GMOs. The measure passed with a 57% majority. In 2005, a standing committee of the government of
Prince Edward Island in Canada began work to assess a proposal to ban the production of GMOs in the
province. This is a largely symbolic and empty gesture as PEI has already banned GM potatoes, which
account for most of its crop. In California, Trinity and Marin counties have also imposed bans on GM crops,
while ordinances to do so were unsuccessful in Butte, San Luis Obispo, Humboldt, and Sonoma counties.
Supervisors in the agriculturally-rich counties of Fresno, Kern, Kings, Solano, Sutter, and Tulare have
passed resolutions supporting the practice [7]
(http://www.santacruzsentinel.com/archive/2005/June/15/local/stories/07local.htm) .
Currently, there is little international consensus regarding the acceptability and effective role of modified
"complete" organisms such as plants or animals. A great deal of the modern research that is illuminating
complex biochemical processes and disease mechanisms makes vast use of genetic engineering.
The practice of genetic modification as a scientific technique is not restricted in the United States. Individual
genetically modified crops (such as soybeans) are subject to intense study before being brought to market
and are common in the United States, but estimates of their market saturation vary widely. Most countries in
Europe, Japan, Mexico (among others) have taken the opposite position, stating that genetic modification
has not been proven safe, and therefore that they will not accept genetically modified food from the United
States or any other country. This issue has been brought before the World Trade Organization, which
determined that not allowing modified food into the country creates an unnecessary obstacle to international
trade. Consequently, genetic modification within agriculture is an issue of some strong debate in the United
States, the European Union, and some other countries.
Some critics have raised the concern that conventionally bred crop plants can be cross-pollinated (bred)
from the pollen of modified plants. Pollen can be dispersed over large areas by wind, animals, and insects.
Recent research with creeping bentgrass has lent support to the concern when modified genes were found in
normal grass up to 21 km (13 miles) away from the source, and also within close relatives of the same genus
4 of 6
23/11/06 15:15
(Agrostis) [8]
(http://www.pnas.org/cgi/content/abstract/101/40/14533?ijkey=e52c2f60e681a753eab3d267158057b81ffc6b13&keytyp
. GM proponents point out that outcrossing, as this process is known as, is not new. The same thing happens
with any new open-pollinated crop variety—newly introduced traits can potentially cross out into
neighbouring crop plants of the same species and, in some cases, to closely related wild relatives. Defenders
of GM technology point out that each GM crop is assessed on a case by case basis to determine if there is any
risk associated with the outcrossing of the GM trait into wild plant populations. The fact that a GM plant may
outcross with a related wild relative is not, in itself, a risk unless such an occurrence has consequences. If, for
example, a herbicide resistance trait was to cross into a wild relative of a crop plant it can be predicted that
this would not have any consequences except in areas where herbicides are sprayed, such as a farm. In such
a setting the farmer can manage this risk by rotating herbicides. If patented genes are outcrossed, even
accidentally, to other commercial fields and a person deliberately selects the outcrossed plants for subsequent
planting then the patent holder has the right to control the use of those crops. This was supported in
Canadian law in the case of Monsanto Canada Inc. v. Schmeiser. The documentary The Future of Food
covers the GMO and Monsanto controversy in more depth.
An often cited controversy is a hypothetical Technology Protection technology (dubbed terminator by
non-governmental organization). This yet-to-be-commercialised technology would allow the production of
first generation crops that would not generate seeds in the second generation because the plants yield sterile
seeds. The patent for this so-called "terminator" gene technology is owned by Delta and Pine Land and the
United States Department of Agriculture. Delta and Pine Land was bought by Monsanto in August 2006. In
addition to the commercial protection of proprietary technology in selfpollinating crops such as soybean (a
generally contentious issue) another purpose of the terminator gene is to prevent the escape of genetically
modified traits from crosspollinating crops into wild-type species by sterilizing any resultant hybrids. The
terminator gene technology created a backlash amongst those who felt the technology would prevent re-use
of seed by farmers growing such terminator varieties in the developing world and was ostensibly a means to
exercise patent claims. Use of the terminator technology would also prevent "volunteers", or crops that grow
from unharvested seed, a major concern that arose during the Starlink debacle.
For more information on this topic see the [Greenpeace][9] (http://weblog.greenpeace.org/ge/) GE website.
Transgenics featured in fiction
Genetically modified characters, whether as heroes, villains, or backdrop, feature prominently in many
works of fiction, in particular science fiction and cyberpunk, where it is used as a plot device to explain
differences in a character or setting, such as explaining increased longevity or eradication of disease in a
fictional civilization.
In the Spider-Man movie, Peter Parker was bitten by a super-spider, enhanced with the genes of many
different spiders. The abilities of all these spiders were then transferred from the super-spider, into Peter,
turning him into Spider-Man.
The videogame character Shadow the Hedgehog was originally a science experiment who was fused with the
DNA of Black Doom, causing him to have the genes of aliens as well as hedgehogs. This however, was not
revealed until the game Shadow the Hedgehog.
Dark Angel deals with transgenics escaped from a secret government facility. The main character Max, a
human / cat transgenic engineered to be a supersoldier in future warefare, tries to lead a normal life while
searching for others of her kind.
5 of 6
23/11/06 15:15
In the Maximum Ride books by James Patterson, the main characters are human/bird transgenics.
In Red Dwarf, mankind turns to GELFs, Genetically Engineered LifeForms, after the robot revolution leaves
them with nothing.
References
Anderson, K. and Lee Ann Jackson. 2005. Some Implications of GM Food Technology Policies for
Sub-Saharan Africa. Journal of African Economies 14(3):385-410; doi:10.1093/jae/eji013
Heong, KL, YH Chen, DE Johnson, GC Jahn, M Hossain, RS Hamilton. 2005. Debate Over a GM Rice Trial
in China. Letters. Science, Vol 310, Issue 5746, 231-233 , 14 October 2005.
Huang, J., Ruifa Hu, Scott Rozelle, Carl Pray. 2005. Insect-Resistant GM Rice in Farmers' Fields: Assessing
Productivity and Health Effects in China. Science (29 April 2005) Vol. 308. no. 5722, pp. 688 – 690. DOI:
10.1126/science.1108972
See also
Genetically modified food
Fish Farming
External links
International Conference on "GM Crops and Foods" (20/21 November in Frankfurt/Germany)
(http://www.akademie-fresenius.de/1824)
Everything you wanted to know about GM organisms
(http://www.newscientist.com/channel/life/gm-food) — Provided by New Scientist.
Genetically Modified Organisms (http://www.organicfooddirectory.com.au/gmo.php) - Information
about GMOS and GE
Eppendorf Biochip Systems (http://www.eppendorf-biochips.com) Detection method for GMO in food
and feed by using GMO-microarray
Nature 2.0 beta | Legislation, Politics, Science and Spin Behind Genetically Modified Foods
(http://thegreenreport.com/nature2.0)
Food Security and Ag-Biotech News (http://www.merid.org/fs-agbiotech/) — for balanced news
Devlin RH, Sundstrom LF, Muir WM. 2006. Interface of biotechnology and ecology for environmental
risk assessments of transgenic fish. Trends in Biotechnology 24:89-97
(http://dx.doi.org/10.1016/j.tibtech.2005.12.008) - A scientific article on the advances and problems in
making reliable risk-assessment of transgenic fish.
Bernard Stiegler, "Take Care" (http://www.arsindustrialis.org/Members/bstiegler/prendresoin-en) — A
philosophical approach to the question of GMOs and their relation to human agricultural history.
Retrieved from "http://en.wikipedia.org/wiki/Genetically_modified_organism"
Categories: Articles lacking sources | Genetically modified organisms | Molecular biology
6 of 6
23/11/06 15:15
Reading(s) #11
Steinbrecher and Mooney, Terminator Technology
http://www.gse.buffalo.edu/FAS/Bromley/classes/socprac/readings/S...
University Libraries-University at Buffalo-State Univ. of NY
Expanded Academic ASAP
The Ecologist, Sept-Oct 1998 v28 n5 p276(4)
Terminator Technology: the threat to world food security. Ricarda A. Steinbrecher;
Pat Roy Mooney.
Author's Abstract: COPYRIGHT 1998 The Ecologist (UK)
Monsanto's latest flagship technology makes a nonsense of its claim that it seeks to feed the worlds
hungry. On the contrary, it threatens to undermine the very basis of traditional agriculture - that of
saying seeds from year to year. What's more, this "gene cocktail" will increase the risk that new toxins
and allergens will make their way into the food chain.
Full Text: COPYRIGHT 1998 The Ecologist (UK)
In 1860, fully five years before Abbe Gregor Mendel published his obscure tome on the genetics of
peas, launching so-called "modern" plant breeding, a certain Major Hallett, F.L.S., of Brighton was
warning farmers and fellow seedsmen that any abuse of his "pedigree" trademark for cereals would
be "severely dealt with".[1] But his seeds were not patentable and there was little he could do to keep
farmers from buying his wheat varieties, sowing them, selecting the best seed for the next season, and
breeding their own varieties uniquely adapted to local soils, slopes, and weather.
It was only in 1908 that George Shull came up with what Major Hallett really wanted - a biological
weapon to keep farmers from saving and developing their own seeds. Called "hybridization", a
wonderfully euphemistic term that led farmers to think that crossing two distant plant relatives could
create a "hybrid vigour" that so improved yield as to make the resulting seed sterility - meaning it
could not be replanted - financially worthwhile.[2] Today, almost every ear of corn grown from
California to Kazakhstan is a hybrid controlled by any one of a handful of very large seed companies.
Exactly 90 years after Shull's revelation, one of the biggest and most powerful of those companies,
Monsanto, is fighting for control of the most important seed monopoly technology since the hybrid.
But unlike 1860, this piece of life control can be patented. On March 3rd, the US Department of
Agriculture (USDA) and a little-known cotton-seed enterprise called Delta and Pine Land Company,
acquired US patent 5,723,765 - or the Technology Protection System (TPS). Within days, the rest of
the world knew TPS as Terminator Technology. Its declared goal is to promulgate plants that will
produce self-terminating off-spring - suicide seeds. Terminator Technology epitomizes what the
genetic engineering of food crops is all about and gives an insight into the driving forces behind the
corporate campaign to control and own life.
The Terminator rides to the rescue of long-suffering multinationals who have been unable to hold
farmers back from their 12,000 year tradition of saving and breeding seeds. Farmers buy the seed
once and do their own work thereafter. Patents and Pinkerton detectives have been employed to stop
farmers from doing so. The Terminator though provides a built-in biological "patent", enforced by
engineered genes. Small farming communities of the Third World especially, rely upon their own
plant breeding since neither corporate nor public breeders show much interest or aptitude in breeding
for their often difficult environments. Old-fashioned hybrids and the Terminator Technology with its
terminated seeds force farmers back to the market every season. Terminator also scuttles community
conservation of agricultural biodiversity. There's nothing to conserve. It is the "neutron bomb" of
agriculture.
1 of 6
23/11/06 15:16
Hybrid seeds
Following the rediscovery of Mendel's Laws in 1900, money-minded plant breeders pursued
strategies that would force farmers back to the marketplace every season to spend their hard-earned
money on seeds. Although the concept of hybrids evolved with George Shull in 1908, the first hybrid
maize was not commercialized until 1924 by Henry A. Wallace. Two years later, Wallace formed
Pioneer Hi-Bred the world's largest seed company and still largely controlled by the founding family.
Wallace went onto become US Secretary of Agriculture and, finally (in 1941), Vice-President of the
United States. Wallace's championship of hybrids made it an immutable, if unscientific, Act of Faith
to argue that "hybrid vigour" made maize the "bin-busting" bonanza it is today. More recently,
however, respected scientific and economic critics like Jean-Pierre Berlan of France's INRA and
Richard C. Lewontin of Harvard, as well as Jack R Kloppenburg Jr. of the University of Wisconsin,
have challenged this assumption insisting that conventional maize-breeding programmes would
always out-perform hybrids given the same research investment. According to these critics, the only
advantage to hybrids lies in their profitability for companies.
How hybrids work
Hybrid seeds are the first generation (F1) progeny of two distinct and distant parental lines of the
same species. The seed will incorporate and express the desired genetic traits of each parent for just
one generation. Seeds taken from an F1 hybrid may either be sterile or, more commonly, fail to "breed
true", not express the desirable genetic qualities found in F1. Farmers in industrialized agricultural
systems rarely attempt to replant a hybrid because of the exacting requirements of machine-harvesting
and food-processing for crop uniformity. Resource-poor farmers in countries such as Brazil, on the
other hand, will often take F2 (second generation) hybrid seeds as a source of breeding material to be
blended with their traditional varieties. In this way, skilled local breeders, mostly women,be they in
Brazil, Burundi or Bangladesh, isolate useful genetic characteristics and adapt them to their immediate
market. The most commonly hybridized crops are maize, cotton, sunflowers and sorghum.
Until recently, small grain cereals such as rice, wheat, barley, oats, and rye and leguminous crops
such as soybeans, have defied such commercial hybridization. Now this is changing. Public breeding
initiatives led by governments such as China and institutions such as the Rockefeller Foundation and
Cornell University have developed commercial rice hybrids. The seed multinationals are hot on their
heels. Most recently, giants like Monsanto and Novartis have been waxing poetic over the prospect of
Fl hybrid wheat. With more land sown to wheat than any other crop on the planet, a new hybrid
monopoly for this crop would be a windfall for seed companies.[3]
Terminator Technology: The Terminator as Biological Warfare on Farmers and Food Security
The Terminator does more than ensure that farmers can't successfully replant their harvested seed. It is
the "platform" upon which companies can load their proprietary genetic traits - patented genes for
herbicide-tolerance or insect-resistance - and get the farmers hooked on their seeds and caught in the
chemical treadmill. The Terminator is a guarantee that even Brazil's innovative farmers will have to
buy access to these traits every year.
The target market for the Terminator is explicitly the South's farmers. Beginning with company news
releases announcing the patent, Delta and Pine has trumpeted that its Technology Protection System
will make it economically safe for seed companies to sell their high-tech varieties in Africa, Asia and
Latin America. The company has even estimated that 405 million hectares will be sown with
Terminator seeds within a few years. This is a land mass almost equal to South Asia. Although
Terminator Technology has only been tested in cotton and tobacco, its designers are convinced that it
can be applied to any species. Delta and Pine has specifically suggested that rice and wheat farmers in
countries like India, China and Pakistan are a priority market. According to the company, Terminator
2 of 6
23/11/06 15:16
Technology's value could run as high as $4.00 per hectare for upmarket garden crops. The patent
could be worth a billion dollars.[4]
"The centuries old practice of farmer-saved seed is really a gross disadvantage to Third World farmers
who inadvertently become locked into obsolete varieties because of their taking the "easy road" and
not planting newer, more productive varieties." - Dr. Harry B. Collins, Delta and Pine Land Co,
Vice-President for Technology Transfer (June 12, 1998)[5]
How the Terminator Technology works
The Terminator Technology is the main application of a broadly framed patent for the "control of plant
gene expression". The Terminator is basically a genetically engineered suicide mechanism that can be
triggered off by a specific outside stimulus. As a result the seeds of the next generation will
self-destruct by self-poisoning. The preferred trigger is the antibiotic tetracycline applied to seeds. The
main version of the Terminator consists of a set of three novel genes inserted into one plant [see Box
1]; another version divides two or three genes on to two plants, which are later to be cross-pollinated.
The end-result is always a dead seed in the following generation.
Terminator Technology is the Trojan Horse for the spread of genetically-engineered crops in the
South. In the absence of "effective" patent regimes, companies can still market their wares and enforce
constant returns for their investments. In the absence of adequate biosafety legislation, countries might
be persuaded to accept the Terminator on the assumption that the technology is safe and that
transgenic traits can not survive to a second generation, even by cross-pollination. This assumption is
ill-founded. As with all genetic engineering, its direct effect and its side-effects are unpredictable and
carry all the risks inherent in this technology. The gene-cocktail of the Terminator increases the risks
that new toxins and allergens will show up in our food and animal fodder.
Most alarming though is the possibility that the Terminator genes themselves could infect the
agricultural gene pool of the neighbour's crops and of wild and weedy relatives, placing a time-bomb.
Temporary "gene silencing" of the poison gene or failed activation of the Terminator countdown
enables such infection [see Box 2].
Between 15 and 20 per cent of the world's food supply is grown by poor farmers who save their
seed. These farmers feed at least 1.4 billion people. The Terminator "protects" companies by risking
the lives of these people. Since Terminator Technology has absolutely zero agronomic benefit, there is
no reason to jeopardize the food security of the poor by gambling with genetic engineering in the
field. Whether the Terminator works immediately or later, in either instance it is biological warfare on
farmers and food security.
The Terminator also portends a hidden dark side. As a Trojan Horse for other transgenic traits, the
technology might also be used to switch any trait off or on. At least in theory, the technology points to
the possibility that crop diseases could be triggered by seed exports that would not have to "kick in"
immediately - or not until activated by specific chemicals or conditions. This form of biological
warfare on people's food and economies is becoming a hot topic in military and security circles.(6)
Terminator meets the "Monster"
Scarcely two months after USDA and Delta & Pine Land announced the receipt of the Terminator
patent, Monsanto bought the company. The announcement of the $1.76 billion purchase came on May
11th even as parties to the Convention on Biological Diversity were meeting in Bratislava. The
Terminator had already elbowed its way into conference debates when press stories reached
delegations. Overnight the US delegation, who had not uttered a word when even the USDA was
under attack for its Terminator involvement, came out fighting for Monsanto. With former Clinton
White House staffers on Monsanto's lobby payroll and Mickey Cantor, the US Trade Representative
3 of 6
23/11/06 15:16
for much of the Uruguay Round, on Monsanto's board, the American government's zeal was less than
surprising. [See Ferrara in this issue]
Seed technology has moved a long way since 1860 and the proprietary passions of Major Hallett.
Short months before the Major trade-marked his pedigreed seed, the keynote speaker to the
Wisconsin agricultural fair warned the farmers and scientists to beware of new technologies that
distance farmers from their crops. Although his immediate concern was the steam engine's use in
agriculture - he wasn't against it, just worried about whose interests it was serving - the speaker
opined that the task of agricultural technology is to provide a decent living for farmers and to feed
people. Clinton's administration might do well to heed Abraham Lincoln's advice before allowing the
Terminator to enslave the world's farmers today.[7]
Terminating the Terminator
People's organizations and governments can halt the Terminator. Legal means are available through
International Law and existing intergovernmental convention to outlaw the technology. Here are a few
possibilities.
1. The USDA/Delta patent is pending around the world. The patent can and should be rejected on the
grounds that it is in conflict with public morality. The Terminator is a threat to food security and
destructive of agricultural biodiversity. On these grounds, governments are fully entitled under the
terms of even the quarrelsome TRIPS chapter of the WTO (World Trade Organization) agreement to
refuse the patent. In doing so, governments are also (according to the WTO) agreeing not to allow the
technology to be exploited by others within their territory.
2. Pressure (within and without the United States) should be put on the USDA to refuse to surrender
the patent to the company. In fact, the USDA (which surprised itself with the March 3rd patent
announcement) should also petition the US Patent and Trademark Office to revisit the claims and
determine whether or not it is indeed in conflict with public morality.
3. The 100+ member states to the Convention on the Prohibition of the Development, Production, and
Stockpiling of Bacteriological and Toxic Weapons, and on Their Destruction (1972) should call for
the abolition of Terminator Technology as a form of economic biological warfare that not only makes
war on farming communities but could be manipulated to threaten national food security and destroy
the national agricultural economy.
4. At its October 1998 meeting the Consultative Group on International Agricultural Research
(CGIAR), the world's largest international public plant breeding network) should announce its
opposition to the Terminator and its refusal to use it itself.
5. At its May 1999 meeting, the Convention on Biological Diversity's Subsidiary Body on Science
and Technology should pass a resolution declaring the Terminator a threat to agricultural biodiversity
and calling for its removal. Such an initiative would strengthen national efforts to ban the patent and
the technology under the terms of the World Trade Agreement.
Box 1. In a Terminator plant, three genes are inserted, each with an associated regulatory switch,
called a 'promoter'. One of these genes, when switched on, produces a protein called Recombinase,
which acts like molecular scissors [ILLUSTRATION FOR FIGURE 1B OMITTED]. The
Recombinase removes a 'spacer' between the toxin-producing gene [ILLUSTRATION FOR
FIGURE 1A OMITTED] and its promoter. While it is there, the spacer acts as a safety catch to
prevent the toxin gene from being activated.
A third gene is engineered to produce a Repressor [ILLUSTRATION FOR FIGURE 1C
4 of 6
23/11/06 15:16
OMITTED] which keeps the Recombinase gene turned off until the plant with the Terminator
Technology is exposed to a specific outside stimulus, such as a particular chemical, temperature
shock, or osmotic shock. When the chosen stimulus is applied to the seed before sale, the functioning
of the Repressor gets interrupted. And as it is no longer repressed, the recombinase gene is switched
on. The Recombinase that is now produced, removes the spacer 'safety catch'. Because the promoter
in front of the toxin gene is chosen to only become active in the late stages of seed maturation, only
then will it initiate the production of the poison that kills the seed.
The preferred genes used in the Terminator Technology are:
For toxin gene R.I.P. gene (ribosomal inhibitor protein) promoter LEA promoter (late embryonic
abundance) spacer a stretch of DNA framed with specific recognition sites (LOX)
For Recombinase gen CRE/LOX system from bacteriophage (viruses that attack bacteria)
promoter a promoter that can be repressed
For Repressor gene Tetracycline repressor system (Tn10 tet)
Box 2. "Gene silencing" was discovered in the early nineties when, in a field of 10,000 petunias
genetically-engineered to carry a uniform red gene, many of the plants were found blooming white
and pink.[1] Plants are capable of deactivating genes and their promoters if recognized as intruders or
as duplicates of their own DNA.[2] Furthermore, genes that have been deactivated can become active
again generations later. The LEA promoter, which is used to regulate Terminator's toxin gene, is very
common among plants and shows significant similarities across many species; once added, the plant
might choose to switch it off. If this were to happen whilst plants were being multiplied for the
commercial market, no one could tell. Seeds of such plants will eventually be treated with tetracycline;
the blocking sequence [ILLUSTRATION FOR FIGURE 1A OMITTED] will be cut out but no toxin
is produced at the end of the life cycle. The pollen carrying the silent but functional toxin gene could
spread into neighbouring cropfields and forests.
Another likely scenario is that some plants will not react to the tetracycline treatment. Consider the
vast quantities of antibiotics necessary to soak millions of seeds. Who is going to check that all seeds
have taken up the chemical, with a generation having to pass before results can be seen? Again pollen
will spread - with all its novel genes. If down the line the Repressor passes onto one plant, but the
toxin and the Recombinase pass to another, all the seeds produced by the second plant would commit
suicide. Even if all three genes stay together, there might be a future chemical input that acts like
tetracycline.
References
1. see Steinbrecher, R.A., 1996. "From Green to Gene Revolution". The Ecologist Vol.26 No.6. pp.
273-281.
2. Kumpatla, S.P. et al. 1998. "Genome intruder scanning and modulation systems and transgene
silencing", Trends in Plant Science. 3(3): pp. 97-104.
Box 3.Tetracycline is a broad spectrum antibiotic. It is used in medicine to kill bacteria, but it can also
affect humans if wrongly used. The soil is full of vital micro-organisms, including bacteria, on which
the health of plants depend. Whilst plants will normally grow up in close partnership with
soil-organisms, the tetracycline-soaked seeds could create a death zone around them, destroying the
fragile balance of the microbial soil web. As a consequence farmers would have to resort to chemicals
to protect their crop from disease and apply fertilizers to make them grow. The Terminator would not
only deplete diversity, but also destroy soil.
5 of 6
23/11/06 15:16
References and Notes
1. Berlan, Jean-Pierre and Richard C..Lewontin. "Agricultural Genetics and Sterifix Breeding" (1998)
unpublished manuscript, from an advertisement inserted at pages 5-6.
2. Lewontin, Richard C., and Berlan, Jean-Pierre, "The Political Economy of Agricultural Research:
The Case of Hybrid Corn" Chapter 23, p. 625 in Carroll, Ronald, C., Vandermeer, John H., and
Rossett, Peter, Agroecology, McGraw-Hill Publishing Co.
3. For further information about the new push for cereal hybrids, please see RAFI Communique
"Seed Industry Consolidation - 1988: Who Owns Whom?" (July/August, 1988) at the RAFI website
www.rafi.ca/
4. Freiburg, Bill, "Is Delta and Pine Land's Terminator Gene a Billion Dollar Discover?" in Seeds and
Crop Digest, May-June, 1998.
5. Collins, Harry B. "New Technologies and Modernizing World Agriculture", an unpublished paper
distributed by Dr. Collins during a debate on Terminator held June 12th, 1998 during the FAO
Commission on Genetic Resources for Food and Agriculture (Rome).
6. Via a Freedom of Information application to the US Army, RAFI recently received documentation
from a military seminar titled, "Biotechnology Workshop 20/20" (May 29-30, 1996, held at the Army
War College. The papers outline a wide range of military uses for biotechnology which the authors
believe to be feasible by the year 2020.
7. Abraham Lincoln, "Annual Address by Hon. Abram Lincoln of Illinois delivered at Milwaukee,
Sept. 30, 1859" pages 287-299 in Transactions of the Wisconsin State Agricultural Society. Carpenter
and Hyer, (Madison) 1860.
Dr. Ricarda A. Steinbrecher is a geneticist and biologist. She is coordinating the Test Tube Harvest
Campaign of the Women's Environmental Network, is Science Director of the Genetics Forum, UK
and is biotechnology advisor to many non-governmental organisations.
Pat Roy Mooney has worked for more than 30 years with civil society organisations on international
trade and development issues related to agriculture and biodiversity and is the author of several books
on the subject. He lives in Winnipeg, Canada, where he is Executive Director of RAFI.
Copyright © 2000, Gale Group. All rights reserved.
Gale Group is a Thomson Corporation Company.
6 of 6
23/11/06 15:16

20.5 M - Thierry Karsenti

Transcription

Similar documents

Karen K. Resendes Westminster College New Wilmington, Pa

10 species of sea anemones

Meiosis

Lecture 11

Mathematical Modeling as a Means to Demonstrate the

Wednesday, March 20 6 to 8 p.m. Centerra Mirage Elementary

Meiosis and Mendel`s Law of Segregation

Computational Systems Biology 1 6106, 5 cr

Cell division

Protecting Animals - Maricopa County Attorney`s Office