EUKARYOTIC CHROMOSOMES, MITOSIS AND MEIOSIS

EUKARYOTIC CHROMOSOMES, MITOSIS
AND MEIOSIS
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Chromosomes contain the genetic
material
Genes are physically located within the
chromosomes
Chromosomes are composed of DNA and
proteins
Primary function of genetic material is to
store needed information
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Eukaryotic chromosomes
Typical chromosome contains a single,
linear, double-stranded DNA molecule
DNA must be folded and packaged
Chromatin is the DNA-protein complex
making up chromosomes
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Compaction
DNA wraps itself around histone proteins
Nucleosome is a repeating unit of DNA
wrapped around an octamer of histone
proteins
Negative charges of DNA attracted to
positive charges of histones
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Compaction
Amino terminal tails of
histone proteins
protrude from octamer
Modification
can control
the degree of compaction
Shortens length of DNA
about sevenfold
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Noll Confirmed Kornberg’s Beads-on-a-String
Model by Digestion of the Linker Region
Beads-on-a-string model of nucleosome structure
was originally proposed by Roger Kornberg in
1974
Noll reasoned that if the model was correct, the
linker region of DNA would be more accessible to
DNase-I than would the 146-bp region that is
tightly bound to the histones
He expected incubation with DNase-I to make
cuts in the linker region and produce DNA pieces
that would be approximately 200 bp in length
HYPOTHESIS: DNA wraps around histone proteins in a regular, repeating pattern
STARTING MATERIAL: Nuclei from rat liver cells
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30-nm fiber
Nucleosome units are organized into a
more compact structure that is 30 nm in
diameter
Shortens the nucleosome structure
another sevenfold
Structure has proven difficult to determine
because the conformation of the DNA may
be substantially altered when extracted
from living cells
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30-nm fibers interact with the nuclear matrix
Compacts 30-nm fiber by participating in the
formation of radial loop domains
Anchored to nuclear matrix
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Each chromosome located in discrete
nonoverlapping chromosome territory
Different chromosomes are not substantially
intertwined even when noncompacted
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Compaction is not uniform
Heterochromatin
Highly
compacted and transcriptionally inactive
Some localized regions in nondividing cells
Euchromation
Less
condensed and capable of gene
transcription
Most chromosomal regions in nondividing cells
Chromosomes compact further when the cell
is preparing to divide
All
euchromatin converted to heterochromatin
Most transcriptional activities cease during cell
division
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In metaphase chromosomes, highly compacted
radial loops remain anchored to a scaffold
formed from proteins in the nuclear matrix
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Mitotic cell division
A cell divides to produce 2 new cells
genetically identical to the original
Original called mother, new cells called
daughters
Involves mitosis and cytokinesis
Can be for asexual reproduction or for
production and maintenance of
multicellularity
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Cytogenetics
Field of genetics involving microscopic
examination of chromosomes
Tightly compacted chromosomes have
distinctive shapes and number of
chromosomes visible with light microscope
Chemical dye gives a distinctive banding
pattern
Karyotype
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Eukaryotic chromosomes occur in sets
Humans
have 23 different types
Sex chromosomes are designated X and Y in
humans
In many species, individual has 2 sets of
chromosomes
Humans-
23 pairs for 46 total chromosomes
Diploid, 2n
Haploid, 1n – gametes have 1 member of
each pair
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When a species is diploid, members of a
pair of chromosomes are called
homologues
One of each of the these pairs comes from
the mother, the other from the father
Very similar, nearly identical in size and
genetic composition
Slight
differences provide variation in gene
function
Sex chromosomes differ in size and
genetic composition
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Preparation for cell division
DNA replicated
Sister chromatids - 2
identical copies with
associated proteins
Tightly associates at
centromere
Serves as attachment
site for kinetochore
used in sorting
chromosomes
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Mitosis
Sorting process that ensures that each
daughter cell will obtain the correct
number and types of chromosomes
Mitotic spindle apparatus (or mitotic
spindle) is responsible for organizing and
sorting the chromosomes during mitosis
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Animal cell mitotic
spindle
Centrosomes
Microtubule
organizing
center (MTOCs)
Single centrosome
duplicates at the
beginning of M phase
Each defines a pole
Animal cells have
centrioles
Not
found in many
other eukaryotes
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Spindle microtubules
Spindle formed from microtubules
Tubulin
proteins
3 types of microtubules
Astral
– position spindle in cell
Polar – separate 2 poles
Kinetochore – attached to kinetochore bound
to centromeres of each chromosome
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Interphase - phase of the cell cycle during
which the chromosomes are decondensed
and found in the nucleus
Mitosis
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis
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Prophase
Chromosomes have already replicated to
produce 12 chromatids, joined as six pairs
of sister chromatids
Nuclear membrane dissociates into small
vesicles
Chromatids condense into highly
compacted structures that are readily
visible by light microscopy
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Prometaphase
Mitotic spindle is completely formed during
this phase
Centrosomes move apart and demarcate
the two poles
Spindle fibers interact with sister
chromatids
Two kinetochores on each pair of sister
chromatids are attached to kinetochore
microtubules from opposite poles
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Metaphase
Pairs of sister chromatids are aligned
along a plane halfway between the poles
called the metaphase plate
Organized into a single row
When this alignment is complete, the cell
is in metaphase
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Anaphase
Connections between the pairs of sister
chromatids are broken
Each chromatid, now an individual
chromosome, is linked to only one of the
two poles by one or more kinetochore
microtubules
Kinetochore microtubules shorten, pulling
the chromosomes toward the pole to
which they are attached
Two poles move farther away from each
other as overlapping polar microtubules
lengthen and push against each other
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Telophase
Chromosomes have reached their
respective poles and decondense
Nuclear membranes now re-form to
produce two separate nuclei
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Cytokinesis
Two nuclei are segregated into separate
daughter cells
Process of cytokinesis is quite different in
animals and plants
Animals
- cleavage furrow constricts like a
drawstring to separate the cells
Plants - cell plate forms a cell wall between
the two daughter cells
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Meiosis
Sexual reproduction requires a fertilization
event in which two haploid gametes unite
to create a diploid cell called a zygote
Meiosis is the process by which haploid
cells are produced from a cell that was
originally diploid
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1.
2.
Like mitosis, meiosis begins after a cell
has progressed through the G1, S, and
G2 phases of the cell cycle
2 key differences
Homologous pairs form a bivalent or
tetrad
Crossing over
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Bivalent or tetrad
Homologous pairs of sister chromatids
associate with each other, lying side by
side to form a bivalent or tetrad
Process called synapsis
Synaptonemal complex not required for
pairing of homologous chromosomes
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Crossing over
Physical exchange between chromosome
pieces of the crossing bivalent
May increase the genetic variation of a
species
Chiasma - arms of the chromosomes tend
to separate but remain adhered at a
crossover site
Number of crossovers carefully controlled
by cells
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Meiosis I
Prophase I – replicated chromosomes
condense and bivalents form as the
nuclear membrane breaks down
Prometaphase I - spindle apparatus
complete, and the chromatids are attached
to kinetochore microtubules
Metaphase I - bivalents organized along
metaphase plate as double row
Mechanism
to promote genetic diversity
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Anaphase I - segregation of homologues occurs
Connections
between bivalents break, but not the
connections that hold sister chromatids together
Each joined pair of chromatids migrates to one pole,
and the homologous pair of chromatids moves to the
opposite pole
Telophase I - sister chromatids have reached
their respective poles and decondense and
nuclear membranes reform
Cytokinesis
Original diploid cell had its chromosomes in
homologous pairs, while the two cells produced
at the end of meiosis I are haploid - they do not
have pairs of homologous chromosomes
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Meiosis II
No S phase between meiosis I and
meiosis II
Sorting events of meiosis II are similar to
those of mitosis
Sister chromatids are separated during
anaphase II, unlike anaphase I
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Meiosis vs. Mitosis
Mitosis produces two diploid daughter
cells that are genetically identical
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chromosomes in 3 homologous pairs
Meiosis produces four haploid daughter
cells
Each
daughter has a random mix of 3
chromosomes
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