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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Mitosis is divided into four phases.
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Interphase, prophase, and metaphase
nuclear
envelope
chromatin
nucleolus
centriole
pairs
(a) Late Interphase
Duplicated chromosomes
are in the relaxed
uncondensed state;
duplicated centrioles
remain clustered.
spindle pole
condensing
chromosomes
beginning of
spindle formation
(b) Early Prophase
Chromosomes condense
and shorten; spindle
microtubules begin to
form between separating
centriole pairs.
spindle
microtubules
kinetochore
spindle pole
(c) Late Prophase The
nucleolus disappears; the
nuclear envelope breaks
down; spindle microtubules
attach to the kinetochore
of each sister chromatid.
(d) Metaphase
Kinetochores interact;
spindle microtubules
line up the
chromosomes
at the cell’s equator.
Fig. 8-9a–d
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Anaphase, telophase, cytokinesis, and
interphase
unattached spindle
microtubules
(e) Anaphase Sister
chromatids separate
and move to opposite
poles of the cell; spindle
microtubules that are
not attached to the
chromosomes push the
poles apart.
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chromosomes
extending
nuclear envelope
re-forming
(f) Telophase One set of
chromosomes reaches
each pole and relaxes
into the extended state;
nuclear envelopes start
to form around each set;
spindle microtubles
begin to disappear.
(g) Cytokinesis
The cell divides in
two; each daughter
cell receives one
nucleus and about
half of the cytoplasm.
(h) Interphase of
daughter cells Spindles
disappear, intact nuclear
envelopes form,
chromosomes extend
completely, and the
nucleolus reappears.
Fig. 8-9e–h
8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 During PROPHASE, the chromosomes condense and
are captured by the spindle microtubules.
 Three major events happen in prophase:
• The duplicated chromosomes condense.
• The spindle microtubules form.
• The chromosomes are captured by the spindle.
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 The centriole pairs migrate with the spindle poles to
opposite sides of the nucleus.
• When the cell divides, each daughter cell receives a
centriole.
 Every sister chromatid has a structure called a
kinetochore located at the centromere, which attaches to
a spindle apparatus.
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 During METAPHASE, the chromosomes line up along the
equator of the cell.
• At this phase, the spindle apparatus lines up the sister
chromatids at the equator, with one kinetochore facing each
cell pole.
Fig. 8-9d
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 During ANAPHASE, sister
chromatids separate and move to
opposite poles of the cell.
• Sister chromatids separate,
becoming independent daughter
chromosomes.
• The kinetochores pull the
chromosomes poleward along the
spindle microtubules.
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 During TELOPHASE, nuclear
envelopes form around both
groups of chromosomes.
• Telophase begins when the
chromosomes reach the poles.
• The spindle microtubules
disintegrate and the nuclear
envelop forms around each group
of chromosomes.
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 CYTOKINESIS occurs during
telophase, separating each daughter
nucleus into a separate cell that then
begins interphase.
 The cytoplasm is divided between two
daughter cells.
• Microfilaments attached to the plasma
membrane form a ring around the
equator of the cell.
• The ring contracts and constricts the
cell’s equator.
• The constriction divides the cytoplasm
into two new daughter cells.
Copyright © 2009 Pearson Education Inc.
8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 CYTOKINESIS
Microfilaments form
a ring around the cell’s
equator.
The microfilament
ring contracts, pinching
in the cell’s “waist.”
(a) Microfilaments contract, pinching the cell in two
The waist
completely
pinches off,
forming two
daughter cells
(b) Scanning electron micrograph
of cytokinesis.
Fig. 8-10
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Cytokinesis in plant cells is different than in
animal cells.
• In plants, carbohydrate-filled vesicles bud off
the Golgi apparatus and line up along the cell’s
equator between the two nuclei.
• The vesicles fuse, forming a cell plate.
• The carbohydrate in the vesicles become the
cell wall between the two daughter cells.
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Cytokinesis in a plant cell
Fig. 8-11
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8.6 How Does Meiotic Cell Division Produce
Haploid Cells?
 Meiosis is the production of haploid cells with
unpaired chromosomes derived from diploid parent
cells with paired chromosomes.
 Meiosis includes two nuclear divisions, known as
meiosis I and meiosis II.
• In meiosis I, homologous chromosomes pair up, but
sister chromatids remain connected to each other.
• In meiosis II, chromosomes behave as they do in
mitosis—sister chromatids separate and are pulled to
opposite poles of the cell.
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
paired homologous
chromosomes
chiasma
spindle
microtubul
e
(a) Prophase I
Duplicated chromosomes
condense. Homologous
chromosomes pair up
and chiasmata occur as
chromatids of homologues
exchange parts by crossing
over. The nuclear envelope
disintegrates, and spindle
microtubules form.
recombined
chromatids
kinetochores
(b) Metaphase I
Paired homologous
chromosomes line up along
the equator of the cell. One
homologue of each pair
faces each pole of the cell
and attaches to the spindle
microtubules via the
kinetochore (blue).
(c) Anaphase I
Homologues separate,
one member of each
pair going to each
pole of the cell. Sister
chromatids do not
separate.
(d) Telophase I
Spindle microtubules disappear.
Two clusters of chromosomes
have formed, each containing
one member of each pair of
homologues. The daughter
nuclei are therefore haploid.
Cytokinesis commonly occurs
at this stage. There is little
or no interphase between
meiosis I and meiosis II.
Fig. 8-12a–d
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
(e) Prophase II
If the chromosomes
have relaxed after
telophase I, they
recondense. Spindle
microtubules re-form
and attach to the
sister chromatids.
(f) Metaphase II
The chromosomes line
up along the equator,
with sister chromatids
of each chromosome
attached to spindle
microtubules that lead
to opposite poles.
(g) Anaphase II
The chromatids separate
into independent
daughter chromosomes,
one former chromatid
moving toward each
pole.
(h) Telophase II
The chromosomes
finish moving to
opposite poles.
Nuclear envelopes
re-form, and the
chromosomes
become extended
again (not shown
here).
(i) Four haploid
cells
Cytokinesis results
in four haploid cells,
each containing one
member of each
pair of homologous
chromosomes
(shown here in the
condensed state).
Fig. 8-12e–i
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8.6 How Does Meiotic Cell Division Produce
Haploid Cells?
 Meiosis I separates homologous
chromosomes into two haploid
daughter nuclei.
• During PROPHASE I, homologues
pair up.
• The two homologues in a pair
intertwine, forming chiasmata
(singular, chiasma).
• At some chiasmata, the homologues
exchange parts in a process known
as crossing over.
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 During METAPHASE I, paired
homologues line up at the
equator of the cell.
• Interactions between the
kinetochores and the spindle
microtubules move the paired
homologues to the equator of
the cell.
Copyright © 2009 Pearson Education Inc.
8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 During ANAPHASE I, homologous
chromosomes separate.
• One duplicated chromosome
(consisting of two sister chromatids)
from each homologous pair moves to
each pole of the dividing cell.
• At the end of anaphase I, the cluster
of chromosomes at each pole
contains one member of each pair of
homologous chromosomes.
Copyright © 2009 Pearson Education Inc.
8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 After TELOPHASE I and
CYTOKINESIS, there are two haploid
daughter cells.
• The spindle microtubules disappear and
the nuclear envelope may reappear.
• Cytokinesis takes place and divides the
cell into two daugher cells; each cell has
only one of each pair of homologous
chromosomes and is haploid.
• Each chromosome still has two sister
chromatids.
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 Meiosis II separates sister chromatids into
four haploid daughter cells.
• It is virtually identical to mitosis, although
it occurs in haploid cells.
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 Prophase II: the spindle microtubules reform
Fig. 8-12e
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 Metaphase II: duplicated chromosomes line
up at the cell’s equator
Fig. 8-12f
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 Anaphase II: sister chromatids move to
opposite poles
Fig. 8-12g
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 Telophase II and cytokinesis: four haploid
cells are formed
Fig. 8-12h–i
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
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8.7 How Do Meiotic Cell Division And
Sexual Reproduction Produce Genetic
Variability?
 Ways to produce genetic variability from
meiotic cell division and sexual
reproduction:
• Shuffling of homologues
• Crossing over
• Fusion of gametes
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8.7 How Do Meiotic Cell Division And Sexual
Reproduction Produce Genetic Variability?
 Shuffling of homologues creates novel
combinations of chromosomes.
• There is a random assortment of homologues to
daughter cells at meiosis I.
• At metaphase I, paired homologues line up at the
cell’s equator.
• Which chromosome faces which pole is
random, so it is random as to which daughter
cell will receive each chromosome.
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8.7 How Do Meiotic Cell Division And Sexual
Reproduction Produce Genetic Variability?
 Random separation of homologues during
meiosis produces genetic variability.
(a) The four possible chromosome arrangements at metaphase
of meiosis I
(b) The eight possible sets of chromosomes after meiosis I
Fig. 8-13
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8.7 How Do Meiotic Cell Division And
Sexual Reproduction Produce Genetic
Variability?
 Crossing over creates chromosomes with novel
combinations of genetic material.
• Exchange of genetic material during prophase I, through
crossing over, is a unique event each time.
• Genetic recombination through crossing over results in the
formation of new combinations of genes on a given
chromosome .
• As a result of genetic recombination, each sperm and each
egg is genetically unique.
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8.7 How Do Meiotic Cell Division And
Sexual Reproduction Produce Genetic
Variability?
 Crossing over
sister
chromatids of
one duplicated
homologue
pair of
homologous
duplicated
chromosomes
chiasmata
(sites of
crossing over)
parts of chromosomes
that have been
exchanged between
homologues
Fig. 8-14
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8.7 How Do Meiotic Cell Division And
Sexual Reproduction Produce Genetic
Variability?
 Fusion of gametes creates genetically variable
offspring.
• Because every egg and sperm are genetically
unique, and it is random as to which sperm
fertilizes which egg, every fertilized egg is also
genetically unique.
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