Cells, cytoplasm, and organelles: (Zellen, Zytoplasma und

Cells, cytoplasm, and organelles: (Zellen, Zytoplasma und

Cells, cytoplasm, and organelles:
(Zellen, Zytoplasma und Organellen)
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Cytoplasm consists of
a gelatinous solution
and contains
microtubules (which
serve as a cell's
cytoskeleton) and
organelles (literally
'little organs')
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Cells, cytoplasm, and organelles
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Microfilaments are fine,
thread-like protein fibers, 3-6
nm in diameter. They are
composed predominantly of a
contractile protein called actin,
which is the most abundant
cellular protein. Microfilaments'
association with the protein
myosin is responsible for
muscle contraction.
Microfilaments can also carry
out cellular movements
including gliding, contraction,
and cytokinesis.
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Microtubules are cylindrical
tubes, 20-25 nm in diameter.
They are composed of subunits
of the protein tubulin. These
subunits are termed alpha and
beta. Microtubules act as a
scaffold to determine cell shape,
and provide a set of "tracks" for
cell organelles and vesicles to
move on. Microtubules also form
the spindle fibers for separating
chromosomes during mitosis.
When arranged in geometric
patterns inside flagella and cilia,
they are used for locomotion.
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Filaments
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Intermediate filaments are about
10 nm diameter and provide tensile
strength for the cell.
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Examples of the cytoskeleton
(in epithelial cells) In the epithelial (skin)
cells of the intestine, all three types of
fibers are present. Microfilaments project
into the villi, giving shape to the cell
surface. Microtubules grow out of the
centrosome to the cell periphery.
Intermediate filaments connect adjacent
cells through desmosomes.
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Cytoskeleton
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The cytoskeleton acts as a
"track" on which cells can move
organelles, chromosomes and
other things. Some examples are:
Vesicle movement between
organelles and the cell surface,
frequently studied in the squid
axon.
Cytoplasmic streaming
Movement of pigment vesicles for
protective coloration
Discharge of vesicle content for
water regulation in protozoa
Cell divisionÆcytokinesis
Movement of chromosomes
during mitosis and meiosis
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Broken motors. In healthy
individuals, the protein
dystrophin is part of the
linkage between the cellular
cytoskeleton and the adhesive
proteins on the outside of the
cell.
In Duchenne Muscular
Dystrophy, however, the gene
that codes for dystrophin is
defective, resulting in muscle
degeneration and finally death.
This disease is X-linked
recessive and occurs in 1 out
of every 3,500 males.
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Cytoskeleton
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Cellular motors Cells have protein motors
that bind two molecules, and using ATP as
energy, cause one molecule to shift in
relationship to the other. Two types of these
protein motors are myosin and actin, and
dynein or kinesin and microtubules.
These families of proteins all have a motor
end, but may have several kinds of different
molecular structures on the binding end.
When these proteins bind, they can cause
many different molecules, organelles, etc. to
move.
To the right is an example of the different
binding ends found in the kinesin family of
motors. When linked to other microtubules,
protein motors can cause motion if the ends
are fixed or extend the lengths of the fiber
bundles if the ends are free.
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External cell movement
(Cellular movement)
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Cellular movement is accomplished by cilia and
flagella. Cilia are hair-like structures that can beat in
synchrony causing the movement of unicellular
paramaecium. Cilia are also found in specialize
linings in eukaryotes. For example, cilia sweep
fluids past stationary cells in the lining of trachea
and tubes of female oviduct.
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Flagella are whip-like appendages that undulate to
move cells. They are longer than cilia, but have
similar internal structures made of microtubules.
Prokaryotic and eukaryotic flagella differ greatly.
Both flagella and cilia have a 9 + 2 arrangement of
microtubules. This arrangement refers to the 9
fused pairs of microtubules on the outside of a
cylinder, and the 2 unfused microtubules in the
center. Dynein "arms" attached to the microtubules
serve as the molecular motors. Defective dynein
arms cause male infertility and also lead to
respiratory tract and sinus problems.
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Cells also contain a nucleus within which is
found DNA (deoxyribonucleic acid) in the form
of chromosomes plus nucleoli (within which
ribosomes are formed)
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The nucleus
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Within the nucleus is the DNA
responsible for providing the cell
with its unique characteristics. The
DNA is similar in every cell of the
body, but depending on the specific
cell type, some genes may be
turned on or off - that's why a liver
cell is different from a muscle cell,
and a muscle cell is different from a
fat cell. When a cell is dividing, the
DNA and surrounding protein
condense into chromosomes (see
photo) that are visible by
microscopy.
The prominent structure in the
nucleus is the nucleolus. The
nucleolus produces ribosomes,
which move out of the nucleus to
positions on the rough endoplasmic
reticulum where they are critical in
protein synthesis.
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The nucleus is the most obvious
organelle in any eukaryotic cell. It is
a membrane-bound organelle and is
surrounded by a double membrane.
It communicates with the
surrounding cytosol via numerous
nuclear pores.
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DNA
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Composition
In eukaryotes, chromosomes
consist of a single molecule of
DNA associated with:
many copies of 5 kinds of
histones.
Histones are proteins rich in lysine
and arginine residues and thus
positively-charged. For this reason
they bind tightly to the negativelycharged phosphates in DNA.
a small number of copies of many
different kinds of non-histone
proteins. Most of these are
transcription factors that
regulate which parts of the DNA
will be transcribed into RNA.
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Structure
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For most of the life of the cell, chromosomes are too
elongated and tenuous to be seen under a
microscope.
Before a cell gets ready to divide by mitosis, each
chromosome is duplicated (during S phase of the
cell cycle).
As mitosis begins, the duplicated chromosomes
condense into short (~ 5 µm) structures which can
be stained and easily observed under the light
microscope.
These duplicated chromosomes are called dyads.
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DNA
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When first seen, the duplicates are held together at their
centromeres. In humans, the centromere contains ~1 million
base pairs of DNA. Most of this is repetitive DNA: short
sequences (e.g., 171 bp) repeated over and over in tandem
arrays.
While they are still attached, it is common to call the duplicated
chromosomes sister chromatids, but this should not obscure
the fact that each is a bona fide chromosome with a full
complement of genes.
The kinetochore is a complex of proteins that forms at each
centromere and serves as the attachment point for the spindle
fibers that will separate the sister chromatids as mitosis
proceeds into anaphase.
The shorter of the two arms extending from the centromere is
called the p arm; the longer is the q arm.
Staining with the trypsin-giemsa method reveals a series of
alternating light and dark bands called G bands.
G bands are numbered and provide "addresses" for the
assignment of gene loci.
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Chromosome
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Chromosome Numbers
All animals have a characteristic number of
chromosomes in their body cells called the diploid
(or 2n) number.
These occur as homologous pairs, one member
of each pair having been acquired from the gamete
of one of the two parents of the individual whose
cells are being examined.
The gametes contain the haploid number (n) of
chromosomes.
(In plants, the haploid stage takes up a larger part
of its life cycle)
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Diploid numbers of some commonly studied
organisms
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Homo sapiens (human)46
Mus musculus (house mouse)40
Drosophila melanogaster (fruit fly)8
Caenorhabditis elegans (microscopic roundworm)12
Saccharomyces cerevisiae (budding yeast)32
Arabidopsis thaliana (plant in the mustard family)10
Xenopus laevis (South African clawed frog)36
Zea mays (corn or maize)20
Muntiacus reevesi (the Chinese muntjac, a deer)23
Muntiacus muntjac (its Indian cousin)6
Myrmecia pilosula (an ant)2
Parascaris equorum var. univalens (parasitic roundworm)2
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Karyotypes
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The complete set of chromosomes in the cells of an organism is its
karyotype. It is most often studied when the cell is at metaphase of
mitosis and all the chromosomes are present as dyads.
The karyotype of the human female contains 23 pairs of
homologous chromosomes:
22 pairs of autosomes
1 pair of X chromosomes
The karyotype of the human male contains:
the same 22 pairs of autosomes
one X chromosome
one Y chromosome
(A gene on the Y chromosome designated SRY is the master switch
for making a male.)
Link to a karyotype of a normal human male stained by the trypsingiemsa method. The X and Y chromosomes are called the sex
chromosomes.)
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Below is a human karyotype (of which sex?). It differs from a
normal human karyotype in having an extra #21 dyad. As a
result, this individual suffered from a developmental disorder
called Down Syndrome. The inheritance of an extra
chromosome, is called trisomy, in this case trisomy 21.
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Translocations
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Karyotype analysis can also reveal translocations between
chromosomes. A number of these cause cancer, for
example
the Philadelphia chromosome (Ph1) formed by a
translocation between chromosomes 9 and 22 and a cause
of Chronic Myelogenous Leukemia (CML)
a translocation between chromosomes 8 and 14 that
causes Burkitt's lymphoma
a translocation between chromosomes 18 and 14 that
causes B-cell leukemia
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DNA Content
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The molecule of DNA in a single human
chromosome ranges in size from 50 x 106
nucleotide pairs in the smallest chromosome
(stretched full-length this molecule would extend
1.7 cm) up to 250 x 106 nucleotide pairs in the
largest (which would extend 8.5 cm).
Stretched end-to-end, the DNA in a single human
diploid cell would extend over 2 meters.
See some of the DNA molecule released from a
single human chromosome. In the intact
chromosome, however, this molecule is packed
into a much more compact structure. The packing
reaches its extreme during mitosis when a typical
chromosome is condensed into a structure about
5 µm long (a 10,000-fold reduction in length).
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Burkitt's Lymphoma
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Burkitt's lymphoma is a solid tumor of B lymphocytes, the lymphocytes that the immune system
uses to make antibodies.
The genes for making antibodies are located on chromosomes 14 (the heavy [H] chains), 2 (kappa
light chains), and 22 (lambda light chains). These genes are expressed only in B lymphocytes
because only B cells have the necessary transcription factors for the promoters and enhancers
needed to turn these antibody genes "on".
In most (approximately 90%) of the cases of Burkitt's lymphoma, a reciprocal translocation has
moved the proto-oncogene c-myc from its normal position on chromosome 8 to a location
close to the enhancers of the antibody heavy chain genes on chromosome 14.
In all the other cases, c-myc has been translocated close to the antibody genes on chromosome 2
or 22. In every case, c-myc now finds itself in a region of vigorous gene transcription, and it may
simply be the overproduction of the c-myc product (a transcription factor essential for mitosis of
mammalian cells) that turns the lymphocyte cancerous. Uncontrolled mitosis of this cell
results in a clone of cancer cells, Burkitt's lymphoma. Many other human cancers involve
chromosome aberrations, such as translocations, at the loci of known proto-oncogenes.
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Endoplasmic Reticulum
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Throughout the eukaryotic cell, especially those responsible for the production
of hormones and other secretory products, is a vast amount of membrane
called the endoplasmic reticulum, or ER for short. The ER membrane is a
continuation of the outer nuclear membrane and its function suggests just how
complex and organized the eukaryotic cell really is.
When viewed by electron microscopy, some areas of the endoplasmic
reticulum look "smooth" (smooth ER) and some appear "rough" (rough ER).
The rough ER appears rough due to the presence of ribosomes on the
membrane surface. Smooth and Rough ER also have different functions.
Smooth ER is important in the synthesis of lipids and membrane proteins.
Rough ER is important in the synthesis of other proteins.
Information coded in DNA sequences in the nucleus is transcribed as
messenger RNA. Messenger RNA exits the nucleus through small pores to
enter the cytoplasm. At the ribosomes on the rough ER, the messenger RNA is
translated into proteins. These proteins are then transferred to the Golgi in
"transport vesicles" where they are further processed and packaged into
lysosomes, peroxisomes, or secretory vesicles.
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Golgi Apparatus
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The Golgi apparatus is a membrane-bound structure
with a double membrane. It is actually a stack of
membrane-bound vesicles that are important in
packaging macromolecules for transport elsewhere in
the cell.
The stack of larger vesicles is surrounded by
numerous smaller vesicles containing those packaged
macromolecules. The enzymatic or hormonal contents
of lysosomes, peroxisomes and secretory vesicles are
packaged in membrane-bound vesicles at the
periphery of the Golgi apparatus.
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Lysosomes, Peroxisomes, Secretory Vesicles
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Lysosomes: (common in animal cells but rare in
plant cells) contain hydrolytic enzymes necessary
for intracellular digestion. In white blood cells that
eat bacteria, lysosome contents are carefully
released into the vacuole around the bacteria and
serve to kill and digest those bacteria.
Uncontrolled release of lysosome contents into
the cytoplasm can also cause cell death
(necrosis).
Peroxisomes: This organelle is responsible for
protecting the cell from its own production of
toxic hydrogen peroxide. As an example, white
blood cells produce hydrogen peroxide to kill
bacteria. The oxidative enzymes in peroxisomes
break down the hydrogen peroxide into water and
oxygen.
Secretory Vesicles: Cell secretions - e.g.
hormones, neurotransmitters - are packaged in
secretory vesicles at the Golgi apparatus. The
secretory vesicles are then transported to the cell
surface for release.
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Phagocytosis is a process describing the
engulfment and destruction of extracellularly-derived materials by phagocytic cells, such as macrophages and
neutrophils. Five steps of phago-cytosis
are illustrated in the image below.
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Phagocytosis of bacteria Schematic
diagram of the steps in phagocytosis:
1.
Attachment of the bacterium to the
long membrane evaginations,
called pseudopodia.
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Ingestion of the bacterium forming
a "phagosome," which moves
toward the lysosome.
3.
Fusion of the lysosome and
phagosome, releasing lysosomal
enzymes into the phagosome.
4.
Digestion of the ingested material.
5.
Release of digestion
Phagocytosis
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Mitochondria
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Mitochondria provide the energy a cell needs to
move, divide, produce secretory products, contract in short, they are the power centers of the cell. They
are about the size of bacteria but may have different
shapes depending on the cell type.
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Mitochondria are membrane-bound organelles, and
like the nucleus have a double membrane. The outer
membrane is fairly smooth. But the inner membrane
is highly convoluted, forming folds called cristae.
The cristae greatly increase the inner membrane's
surface area. It is on these cristae that food (sugar)
is combined with oxygen to produce ATP - the
primary energy source for the cell.
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Mitochondria
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have a double-membrane: outer membrane & highly convoluted
inner membrane
inner membrane has folds or shelf-like structures called cristae that
contain elementary particles; these particles contain enzymes
important in ATP production
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primary function is production of adenosine triphosphate (ATP)
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Ribosomes
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composed of rRNA (ribosomal RNA) & protein
may be dispersed randomly throughout the
cytoplasm or attached to surface of rough
endoplasmic reticulum often linked together in
chains called polyribosomes or polysomes
primary function is to produce proteins
Ribosome
Structure - non-membraneous, spherical bodies
composed of RNA (ribonucleic acid) and protein
enzymes Function - site of protein synthesis
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The Centrosome and the Centrioles
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ANIMAL CELL CENTROSOME: The centrosome, also called the
"microtubule organizing center", is an area in the cell where
microtubles are produced. Within an animal cell centrosome there is a
pair of small organelles, the centrioles, each made up of a ring of nine
groups of microtubules. There are three fused in each group. The two
centrioles are arranged such that one is perpendicular to the other.
During animal cell division, the centrosome divides and the centrioles
replicate (make new copies). The result is two centrosomes, each with
its own pair of centrioles. The two centrosomes move to opposite ends
of the nucleus, and from each centrosome, microtubules grow into a
"spindle" which is responsible for separating replicated chromosomes
into the two daughter cells.
PLANT CELL CENTROSOME: Plant cells have centrosomes that
function much like animal cell centrosomes. However, unlike
centrosomes in animal cells, they do not have centrioles.
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Flagella & cilia
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Smoking?
We know that smoking damages the cilia lining the lungs. As a
result, a smoker’s lungs are not as effective at sweeping dust and
bacteria out of the lungs. This animation demostrates how the cilia in
the lungs of a non-smoker protect the lung.
These tiny hair-like structures lining the inside of the bronchial tubes
are constantly engaged in this sweeping motion, moving dust,
bacteria, and viruses up and out of the lungs. Compare this to the
cilia action inside the lungs of a smoker.
Since the smoker’s lungs are not as effective at sweeping dust,
viruses, and bacteria up and out of the lungs, the smoker is more
susceptible to frequent lung infections.
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Villi
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Projections of cell membrane that serve to
increase surface area of a cell (which is
important, for example, for cells that line
the intestine)
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