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Mitosis & Meiosis
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The division of a unicellular organism reproduces
an entire organism, increasing the population.
Cell division on a larger scale can produce progeny
for some multicellular organisms.
This includes organisms
that can grow by cuttings
or by fission.
Cell d
ivision functions in reproduction,
growth, and repair
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.1
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Cell division is also central to the development of
a multicellular organism that begins as a fertilized
egg or zygote. Multicellular organisms also use cell division to
repair and renew cells that die from normal wear
and tear or accidents.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.1b Fig. 12.1c
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Cell division requires the distribution of identical
genetic material - DNA - to two daughter cells.
What is remarkable is the fidelity with which DNA ispassed along, without dilution, from one generation
to the next.
A dividing cell duplicates its DNA, allocates the
two copies to opposite ends of the cell, and then
splits into two daughter cells.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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genome--A cells genetic information, packaged as
DNA
prokaryotes--often a single long DNA molecule.
eukaryotes--consists of several DNA molecules.
A human cell must duplicate about 3 m of DNA
and separate the two copies such that each
daughter cell ends up with a complete genome.
Cell division distributes identical sets of
chromosomes to daughter cells
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Each eukaryotic chromosome consists of a long,linear DNA molecule.
Each chromosome has hundreds or thousands ofgenes, the units that specify an organismsinherited traits.
Associated with DNA are proteins called
histones that maintain its structure and helpcontrol gene activity.
This DNA-protein complex, chromatin,is
organized into a long thin fiber.After the DNA duplication, chromatin
condenses, coiling and folding to make a smallerpackage.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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DNA is packaged as a chromosome
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Each duplicated chromosome consists of twosister chromatidswhich contain identical copiesof the chromosomes DNA.
As they condense, theregion where the strandsconnect shrinks. This
narrow area, is thecentromere.
Later, the sister
chromatids are pulledapart and repackagedinto two new nuclei atopposite ends of
the parent cell.Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 12.3
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Why do cells divide?
The frequency of cell division varies with cell
type.
Some human cells divide frequently throughout life
(skin cells), others have the ability to divide, but keepit in reserve (liver cells), and mature nerve andmuscle cells do not appear to divide at all aftermaturity.
Cancer cells escape the controls that direct cell
division
Th i i h l i h
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The mitotic(M)phaseof the cell cycle alternateswith the much longer interphase.
The M phase includes mitosis and cytokinesis.
Interphase accountsfor 90% of the cell
cycle.
The mitotic phase alternates with
interphase in the cell cycle: an overview
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.4
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During interphase the cell grows by producing
proteins and cytoplasmic organelles, copies its
chromosomes, and prepares for cell division. Interphase has three subphases:
the G1phase (first gap) centered on growth,
the S phase (synthesis) when DNA replicationoccurs thus the chromosomes are copied,
the G2phase (second gap) where the cell completes
preparations for cell division,
and divides (M).
The daughter cells may then repeat the cycle.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Mitosis is a continuum of changes.
For description, mitosis is usually broken into five
subphases:
prophase
prometaphase
metaphase
anaphase telophase
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
FIVE PHASES
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Late Interphase
By late interphase, the
chromosomes have been
duplicated but are loosely
packed. The centrosomeshave
been duplicated and
begin to organize
microtubules into anaster (star).
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CLICK TO VIEW ANIMATION.
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Prophase
The chromosomes are
tightly coiled, with sister
chromatids joined together.
The nucleoli disappear. The mitotic spindle begins
to form and appears to push
the centrosomes away
from each other toward
opposite ends (poles)
of the cell.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CLICK TO VIEW ANIMATION.
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Prometaphase
The nuclear envelope
fragments and microtubules
from the spindle interact
with the chromosomes. Microtubules from one
pole attach to one of two
kinetochores, special
regions of the centromere,while microtubules from
the other pole attach to
the other kinetochore.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CLICK TO VIEW ANIMATION.
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Metaphase
The spindle fibers push
the sister chromatids
until they are all arranged
at the metaphase plate,an imaginary plane
equidistant between the
poles, defining
metaphase.
CLICK TO VIEW ANIMATION.
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Anaphase
The centromeres divide,separating the sister
chromatids.
Each is now pulledtoward the pole to which
it is attached by spindle
fibers.
By the end, the two
poles have equivalent
collections of
chromosomes.
CLICK TO VIEW ANIMATION.
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Telophase
The cell continues to elongateas free spindle fibers from eachcentrosome push off eachother.
Two nuclei begin for form,surrounded by the fragmentsof the parents nuclearenvelope.
Chromatin becomes
less tightly coiled.
Cytokinesis, divisionof the cytoplasm,begins.
CLICK TO VIEW ANIMATION.
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Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.5 left
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Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.5 right
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Click for a time lapse movie of
ANIMAL mitosis
CLICK TO VIEW ANIMATION.
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More about that spindle apparatus
The mitotic spindle, fibers composed of
microtubules and associated proteins, is a major
driving force in mitosis.
As the spindle assembles during prophase, the
elements come from partial disassembly of the
cytoskeleton.
The spindle fibers elongate by incorporating more
subunits of the protein tubulin.
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Assembly of the spindle microtubules starts in
the centrosome.
The centrosome (microtubule-organizing center) ofanimals has a pair of centrioles at the center, but the
function of the centrioles is somewhat undefined.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.6a
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Each sister chromatid has a kinetochoreof
proteins and chromosomal DNA at the
centromere.The kinetochores of the joined sister chromatids
face in opposite directions.
During prometaphase,some spindle
microtubules
attach to the
kinetochores.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.6b
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When a chromosomes kinetochore is captured
by microtubules, the chromosome moves toward
the pole from which those microtubules come.When microtubules attach to the other pole, this
movement stops and a tug-of-war ensues.
Eventually, the chromosome settles midwaybetween the two poles of the cell, the
metaphase plate.
Other microtubules from opposite poles interactas well, elongating the cell.
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One hypothesis for the movement of
chromosomes in anaphase is that motor proteins
at the kinetochore walk the attachedchromosome along the microtubule toward the
opposite pole.
The excess microtubule sections depolymerize.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.7a
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Experiments
support the
hypothesis thatspindle fibers
shorten during
anaphase from the
end attached to the
chromosome, not
the centrosome.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.7b
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Nonkinetichore microtubules are responsible for
lengthening the cell along the axis defined by the
poles.These microtubules interdigitate across the metaphase
plate.
During anaphase motor proteins push microtubules
from opposite sides away from each other.
At the same time, the addition of new tubulin
monomers extends their length.
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Cytokinesis, division ofthe cytoplasm, typically
follows mitosis.
In animals, the first signof cytokinesis (cleavage)
is the appearance of a
cleavage furrowin thecell surface near the old
metaphase plate.
Cytokinesis divides the cytoplasm
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.8a
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On the cytoplasmic side
of the cleavage furrow a
contractile ring of actinmicrofilaments and the
motor protein myosin
form.
Contraction of the ring
pinches the cell in two.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.8a
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Cytokinesis
Once the nucleardivision has ended, thesecond stage of the Mphase begins
Cytokinesis is separatefrom mitosis [nucleardivision]
Cytokinesis divides thecytoplasm, divvies up theorganelles ANDcompletes the cellsdivision
CLICK TO VIEW ANIMATION.
Mi i i k h l d
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Prokaryotes reproduce by binary fission,notmitosis.
Most bacterial genes are located on a single bacterial
chromosomewhich consists of a circular DNAmolecule and associated proteins.
While bacteria do not have as many genes or DNA
molecules as long as those in eukaryotes, theircircular chromosome is still highly folded and
coiled in the cell.
Mitosis in eukaryotes may have evolved
from binary fission in bacteria
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In binary fission chromosome
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Cell division involves
inward growth of the
plasma membrane,
dividing the parent cellinto two daughter cells,
each with a complete
genome.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 12.10
In binary fission, chromosomereplication begins at one pointin the circular chromosome, the
origin of replicationsite.These copied regions begin to
move to opposite ends of thecell.
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It is quite a jump from
binary fission to
mitosis.
Possible intermediate
evolutionary steps are
seen in the division of
two types of unicellularalgae.
In dinoflagellates,
replicated chromosomes
are attached to thenuclear envelope.
In diatoms, the spindle
develops within the
nucleus.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Chemical signals in the cytoplasm
control the cell cycle!
The cell cycle appears to be driven by specific
chemical signals in the cytoplasm.
Fusion of an S phase and a G1 phase cell, induces the G1
nucleus to start S phase.
Fusion of a cell in mitosis with one in interphase induces
the second cell to enter mitosis.
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The distinct events of the cell cycle are directed
by a distinct cell cycle control system.
These molecules trigger and coordinate key events inthe cell cycle.
The control cycle has
a built-in clock, but it
is also regulated byexternal adjustments
and internal controls.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.13
The G1 Checkpoint the restriction
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If the cells receives a go-ahead signal, it usually
completes the cell cycle and divides. If it does not receive a go-ahead signal, the cell exits
the cycle and switches to a nondividing state, the G0
phase.
Most human cells are in this phase.
Liver cells can be called back to the cell cycle by external
cues (growth factors), but highly specialized nerve and
muscle cells never divide.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The G1 Checkpoint, the restriction
point in mammalian cells is the most
important
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Rhythmic fluctuations in the abundance andactivity of control molecules pace the cell cycle.
Some molecules are protein kinases that activate or
deactivate other proteins by phosphorylating them.The levels of these kinases are present in constant
amounts, but these kinases require a second
protein, a cyclin, to become activated.
Level of cyclin proteins fluctuate cyclically.
The complex of kinases and cyclin forms cyclin-
dependent kinases(Cdks).
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Cyclins activate kinases
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Control of the Cell Cycle
CLICK TO VIEW ANIMATION.
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Cyclin levels rise sharply throughout interphase,
then fall abruptly during mitosis.
Peaks in the activity of one cyclin-Cdk complex,MPF, correspond to peaks in cyclin
concentration.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.14a
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The M phase checkpoint ensures that all the
chromosomes are properly attached to the
spindle at the metaphase plate before anaphase.This ensures that daughter cells do not end up with
missing or extra chromosomes.
A signal to delay anaphase originates at
kinetochores that have not yet attached to spindle
microtubules.
This keeps the anaphase-promoting complex (APC)
in an inactive state.When all kinetochores are attached, the APC
activates, triggering breakdown of cyclin andinactivation of proteins uniting sister chromatids
together.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
A i f l h i l d h i l
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A variety of external chemical and physicalfactors can influence cell division.
Particularly important for mammalian cells aregrowth factors, proteins released by one groupof cells that stimulate other cells to divide.
For example,platelet-derived growth factors (PDGF),
produced by platelet blood cells, bind to tyrosine-kinase receptors of fibroblasts, a type of connectivetissue cell.
This triggers a signal-transduction pathway that leads
to cell division. Each cell type probably responds specifically to a
certain growth factor or combination of factors.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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The role of PDGF is easily seen in cell culture.
Fibroblasts in culture will only divide in the presenceof medium that also contains PDGF.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 12.15
f b k
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Growth factors appear to be a key in density-
dependent inhibitionof cell division.
Cultured cells normallydivide until they form asingle layer on the innersurface of the culturecontainer.
If a gap is created, thecells will grow to fillthe gap.
At high densities, theamount of growth factorsand nutrients is insuffi-cient to allow continuedcell growth.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 12.16a
i l ll l hibi h
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Most animal cells also exhibit anchorage
dependencefor cell division.
To divide they must be anchored to a substratum,typically the extracellular matrix of a tissue.
Control appears to be mediated by connections
between the extracellular matrix and plasma membrane
proteins and cytoskeletal elements.
Cancer cells are free of both density-dependent
inhibition and anchorage dependence.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.16b
Cancer cells have escaped from cell
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Cancer cells divide excessively and invade othertissues because they are free of the bodys control
mechanisms.
Cancer cells do not stop dividing when growth factorsare depleted either because they manufacture their own,
have an abnormality in the signaling pathway, or have a
problem in the cell cycle control system.
If and when cancer cells stop dividing, they do so
at random points, not at the normal checkpoints in
the cell cycle.
Cancer cells have escaped from cellcycle controls
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
C ll di id i d fi i l if h h
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Cancer cell may divide indefinitely if they have a
continual supply of nutrients.
In contrast, nearly all mammalian cells divide 20 to 50times under culture conditions before they stop, age,
and die.
Cancer cells may be immortal.
Cells (HeLa) from a tumor removed from a woman(Henrietta Lacks) in 1951 are still reproducing in culture.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Th b l b h i f ll b i
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The abnormal behavior of cancer cells begins
when a single cell in a tissue undergoes a
transformationthat converts it from a normalcell to a cancer cell.
Normally, the immune system recognizes and
destroys transformed cells.
However, cells that evade destruction proliferate to
form a tumor, a mass of abnormal cells.
If the abnormal cells remain at the originating
site, the lump is called a benign tumor. Most do not cause serious problems and can be
removed by surgery.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
I li h ll l h i i l
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In a malignant tumor, the cells leave the original
site to impair the functions of one or more
organs.This typically fits the colloquial definition of cancer.
In addition to chromosomal and metabolic
abnormalities, cancer cells often lose attachment to
nearby cells, are carried by the blood and lymphsystem to other tissues, and start more tumors in a
event called metastasis.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.17
T f i i i l d
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Treatments for metastasizing cancers include
high-energy radiation and chemotherapy with
toxic drugs.These treatments target actively dividing cells.
Researchers are beginning to understand how a
normal cell is transformed into a cancer cell.The causes are diverse.
However, cellular transformation always involves the
alteration of genes that influence the cell cycle control
system.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Meiosis creates Genetic Diversity
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Purpose of Meiosis
To make haploid cells
When 2 haploid cells fuse; the diploid state is
restored.
You get 1 copy of each chromosome from each
parent. This is called a homologous pair.
This increases genetic diversity.
Karyotypes sort the chromosomes
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y ypinto homologous pairs
[22 + XX or XY]
C iti l diff b t it i
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Critical differences between mitosis
and meiosis
Meiosis reduces chromosome
number by copying the
chromosomes once, but
dividing twice.
The first division, meiosis I,separates homologous
chromosomes.
The second, meiosis II,
separates sister chromatids.
Di ision in meiosis I occ rs in fo r phases
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Division in meiosis I occurs in four phases:
prophase, metaphase, anaphase, and telophase.
During the preceding interphase thechromosomes are replicated to form sister
chromatids.
In prophase I the chromosomes condense and
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In prophase I, the chromosomes condense and
homologous chromosomes pair up to form
tetrads. In a process called synapsis, special proteins attach
homologous chromosomes tightly together.
At several sites the chromatids of
homologous chromosomes arecrossed (chiasmata) and segments
of the chromosomes are traded.
A spindle forms from eachcentrosome and spindle fibers
attached to kinetochores on
the chromosomes begin to
move the tetrads around.
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Crossing Over
CLICK TO VIEW ANIMATION.
At metaphase I the tetrads are all arranged at the
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At metaphase I, the tetrads are all arranged at the
metaphase plate.
Microtubules from one pole are attached to thekinetochore of one chromosome of each tetrad,
while those from the other pole are attached to the
other.
In anaphase I,the homologous
chromosomes
separate andare pulled toward
opposite poles.
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In telophase I movement of homologous
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In telophase I, movement of homologous
chromosomes continues until there is a haploid
set at each pole.
Each chromosome consists of linked sister
chromatids.
Cytokinesis by the same
mechanisms as mitosis
usually occurs simultaneously.
In some species, nuclei
may reform, but there isno further replication
of chromosomes.
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The second meiotic division is
very similar to mitosis.
The 3 critical differences all occurin the first round of meiotic
divisions.
Three critical differencesall occur
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1. Prophase I--homologous chromosomes pair
up in a process called synapsis.
A protein zipper, the synaptonemal complex, holdshomologous chromosomes together tightly.
Later in prophase I, the joined homologous
chromosomes are visible as a tetrad.
At X-shaped regions called chiasmata, sections of
nonsister chromatids are exchanged.
Chiasmata is the physical manifestation of crossing
over, a form of genetic rearrangement.
Three critical differencesall occur
during Meiosis I
2 Metaphase I--homologous pairs of
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2. Metaphase I--homologous pairs of
chromosomes, not individual chromosomes are
aligned along the metaphase plate. In humans, you would see 23 tetrads.
3. Anaphase I--it is homologous chromosomes,
not sister chromatids, that separate and are
carried to opposite poles of the cell.
Sister chromatids remain attached at the centromere
until anaphase II.
The processes during the second meiotic division
are virtually identical to those of mitosis.
Mitosis produces 2 identical cells
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pwhile meiosis produces 4 haploid
and very different cells
Comparison Summary
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Comparison Summary
Worth MAJOR POINTS!
Independent assortment
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Independent assortment
alone would find each
individual chromosome in
a gamete that would be
exclusively maternal or
paternal in origin.
However, crossing over
produces recombinant
chromosomeswhich
combine genes inheritedfrom each parent.
The three sources of genetic variability in a
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T e t ee so ces o ge et c va ab ty asexually reproducing organism are:
Independent assortment of homologous
chromosomes during meiosis I and of nonidenticalsister chromatids during meiosis II.
Crossing over between homologous chromosomesduring prophase I.
Random fertilization of an ovum by a sperm.
All three mechanisms reshuffle the various genescarried by individual members of a population.
Mutations, still to be discussed, are whatultimately create a populations diversity of genes.
Oogenesis vs Spermatogenesis
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Oogenesis vs Spermatogenesis
Oogeneisisegg production
When a girl is born she is born with all her eggs suspended inmeiosis I. Each division of meiosis is uneven. At the end
you have 1 large cell that will be the released egg, and 3 polar
bodies that will disintegrate. Egg production will continue
once a month until the eggs are gone. Spermatogenesissperm production
Once puberty is reached, males continually produce
sperm at a rate of 10s of millions. All cells
produced as a result of meiosis will become sperm.
The human male can release 100 to 650 million
sperm with each ejaculation, and can ejaculate daily
with out losing fertilizing capicity.
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Animal Life Cycle
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Plant Life Cycle