17
The Cell Cycle
17 The Cell Cycle
• The Eukaryotic Cell Cycle
• Regulators of Cell Cycle Progression
• The Events of M Phase
• Meiosis and Fertilization
Introduction
Self-reproduction is perhaps the most
fundamental characteristic of cells.
All cells reproduce by dividing in two,
each parental cell gives rise to two
daughter cells on completion of a cycle
of cell division.
Cell division must be carefully regulated
and coordinated.
Introduction
In eukaryotic cells, progression through
the cell cycle is controlled by protein
kinases that have been conserved from
yeasts to mammals.
Defects in cell cycle regulation are a
common cause of the abnormal
proliferation of cancer cells.
The Eukaryotic Cell Cycle
The division cycle of most cells consists
of four coordinated processes:
• Cell growth
• DNA replication
• Distribution of the duplicated
chromosomes to daughter cells
• Cell division
The Eukaryotic Cell Cycle
In bacteria, cell growth and DNA
replication take place throughout most
of the cell cycle.
Duplicated chromosomes are distributed
to daughter cells in association with the
plasma membrane.
The Eukaryotic Cell Cycle
In eukaryotes, the cell cycle is more
complex. It has four phases: M, G1, S,
and G2.
• M phase: Mitosis (nuclear division),
usually ending with cell division
(cytokinesis).
• Interphase: period between
mitoses, divided into G1, S, and G2.
The Eukaryotic Cell Cycle
• G1 phase (gap 1): interval between
mitosis and DNA replication. The cell is
metabolically active and growing.
• S phase (synthesis): DNA replication
takes place.
• G2 phase (gap 2): cell growth
continues; proteins are synthesized in
preparation for mitosis.
Figure 17.1 Phases of the cell cycle
The Eukaryotic Cell Cycle
Duration of phases varies considerably
in different kinds of cells.
Budding yeasts can progress through all
four phases in 90 minutes.
Early embryos may have cell cycles of
30 minutes, but there is no growth (G1
or G2) phase.
Figure 17.2 Embryonic cell cycles
The Eukaryotic Cell Cycle
In contrast, some cells in adult animals
cease division altogether (e.g., nerve
cells).
Others may divide only occasionally, to
replace cells that have been lost.
The Eukaryotic Cell Cycle
Cell cycle analysis requires identification
of the phases.
Phases of interphase must be identified
biochemically, usually by DNA content.
Animal cells in G1 are diploid (two copies
of each chromosome). Their DNA
content is 2n.
The Eukaryotic Cell Cycle
During S phase, replication increases
the DNA content to 4n.
DNA content can be determined by
incubation of cells with a fluorescent
dye that binds to DNA.
Fluorescence intensity of individual cells
is measured in a flow cytometer or
fluorescence-activated cell sorter.
Figure 17.3 Determination of cellular DNA content
The Eukaryotic Cell Cycle
Progression of cells through the division
cycle is regulated by both extracellular
and internal signals.
Cellular processes, such as growth,
DNA replication, and mitosis, are
regulated by a series of control points.
The Eukaryotic Cell Cycle
A major control point called START
controls progression from G1 to S, first
defined in yeast cells.
Once cells pass START, they are
committed to entering S phase and
undergoing one division cycle.
The Eukaryotic Cell Cycle
Passage through START is highly
regulated by external signals, such as
nutrient availability and cell size.
If there is a shortage of nutrients, yeast
cells can arrest the cycle at START and
enter a resting phase.
Figure 17.4 Regulation of the cell cycle of budding yeast (Part 1)
The Eukaryotic Cell Cycle
In order to maintain constant size, yeast
cells must reach a minimum size to
pass START.
The small daughter cells of budding
yeasts spend a longer time in G1 and
grow more than the large mother cell.
Figure 17.4 Regulation of the cell cycle of budding yeast (Part 2)
The Eukaryotic Cell Cycle
In most animal cells, the restriction
point in late G1 functions like START.
Passage through the restriction point is
regulated by extracellular growth
factors.
Once it has passed the restriction point,
the cell is committed to proceed through
S phase and the rest of the cell cycle.
The Eukaryotic Cell Cycle
If appropriate growth factors are not
present in G1, progression stops at the
restriction point and cells enter a
resting stage called G0.
Skin fibroblasts are arrested in G0 until
stimulated by platelet-derived growth
factor to proliferate and repair wound
damage.
Figure 17.5 Regulation of animal cell cycles by growth factors
The Eukaryotic Cell Cycle
Some cell cycles are controlled in G2.
The fission yeast Schizosaccharomyces
pombe cell cycle is controlled by
transition from G2 to M, the point at
which cell size and nutrient availability
are monitored.
Figure 17.6 Cell cycle of fission yeast (Part 1)
Figure 17.6 Cell cycle of fission yeast (Part 2)
The Eukaryotic Cell Cycle
Cell cycle control in G2 also occurs in
animal oocytes.
Vertebrate oocytes can remain arrested
in G2 for long periods (decades in
humans).
Progression to M phase is triggered by
hormonal stimulation.
The Eukaryotic Cell Cycle
Events in different stages of the cell
cycle must be coordinated so they
occur in appropriate order.
It is critically important, for example, that
the cell not begin mitosis until
replication of the genome has been
completed.
The Eukaryotic Cell Cycle
Coordination of the cell cycle phases is
dependent on a series of cell cycle
checkpoints.
They prevent entry into the next phase
until events of the preceding phase
have been completed.
The Eukaryotic Cell Cycle
DNA damage checkpoints ensure that
damaged DNA is not replicated and
passed on to daughter cells.
The cell cycle is arrested until DNA is
repaired or replicated.
Spindle assembly checkpoint: stops
mitosis at metaphase if chromosomes
are not properly aligned on the spindle.
Figure 17.7 Cell cycle checkpoints
Regulators of Cell Cycle Progression
Recent studies have shown that
eukaryote cell cycles are controlled by
a conserved set of protein kinases,
which trigger the major cell cycle
transitions.
Regulators of Cell Cycle Progression
Three experimental approaches
contributed to identification of the cell
cycle regulators:
1. Studies of frog oocytes, which are
arrested in G2 until hormonal
stimulation triggers entry into M phase.
Regulators of Cell Cycle Progression
In 1971, researchers found that oocytes
could be induced to enter M phase by
microinjection of cytoplasm from
oocytes that had been hormonally
stimulated.
The cytoplasmic factor responsible was
called maturation promoting factor
(MPF).
Figure 17.8 Identification of MPF
Key Experiment, Ch. 17, p. 658 (3)
Regulators of Cell Cycle Progression
Later work showed that MPF is also
present in somatic cells, where it
induces entry into M phase.
MPF thus appeared to act as a general
regulator of the transition from G2 to M.
Regulators of Cell Cycle Progression
2. Genetic analyses of yeasts:
Investigators found temperaturesensitive mutants that were defective in
cell cycle progression (cdc for cell
division cycle mutants).
cdc genes are required for passage
through START and entry into mitosis;
they encode protein kinases.
Figure 17.9 Properties of S. cerevisiae cdc28 mutants
Regulators of Cell Cycle Progression
Related genes were then identified in
other eukaryotes.
The protein kinase has since been
shown to be a cell cycle regulator
conserved in all eukaryotes, known as
Cdk1.
Regulators of Cell Cycle Progression
3. Protein synthesis in early sea urchin
embryos:
In 1983, Hunt and colleagues identified
two proteins (cyclins) that accumulate
throughout interphase but are rapidly
degraded at the end of each mitosis,
suggesting a role in inducing mitosis.
Figure 17.10 Accumulation and degradation of cyclins in sea urchin embryos
Key Experiment, Ch. 17, p. 661 (2)
Regulators of Cell Cycle Progression
Later studies showed that microinjection
of cyclin A into frog oocytes is sufficient
to trigger the G2 to M transition.
Regulators of Cell Cycle Progression
The three experimental approaches
converged in 1988, when MPF was
purified and shown to be composed of
Cdk1 and cyclin B.
Cyclin B is a regulatory subunit required
for catalytic activity of the Cdk1 protein
kinase.
Figure 17.11 Structure of MPF
Regulators of Cell Cycle Progression
Further studies demonstrated the
regulation of MPF by phosphorylation
and dephosphorylation of Cdk1.
During G2, cyclin B is synthesized and
forms complexes with Cdk1.
Cdk1 is phosphorylated and inhibited,
leading to accumulation of inactive
Cdk1/cyclin B complexes during G2.
Regulators of Cell Cycle Progression
Dephosphorylation activates Cdk1,
which phosphorylates several proteins
that initiate the events of M phase.
Cyclin B is degraded by ubiquitinmediated proteolysis.
Destruction of cyclin B inactivates Cdk1,
leading the cell to exit mitosis, undergo
cytokinesis, and return to interphase.
Figure 17.12 MPF regulation
Regulators of Cell Cycle Progression
Ubiquitylation of cyclin B is mediated by
a ubiquitin ligase: the anaphasepromoting complex/cyclosome
(APC/C), which is activated as a result
of phosphorylation by Cdk1/cyclin B.
Regulators of Cell Cycle Progression
Further research has established that
Cdk1 and cyclin B are members of
protein families.
Different members of these families
control progression through the phases
of the cell cycle.
Regulators of Cell Cycle Progression
In yeasts, CDK1 controls G2 to M
transition in association with mitotic Btype cyclins.
Cdk1 controls passage through START
and entry into mitosis in association
with G1 cyclins or Cln’s.
Regulators of Cell Cycle Progression
In higher eukaryotes, there are multiple
cyclins and multiple Cdk1-related
protein kinases, known as Cdk’s for
cyclin-dependent kinases.
Figure 17.13 Complexes of cyclins and cyclin-dependent kinases
Regulators of Cell Cycle Progression
Studies of Cdk’s and cyclins in
genetically modified mice reveal a high
level of plasticity, allowing different
cyclins and Cdk’s to compensate for
the loss of one another.
Cdk1 is capable of substituting for the all
the other Cdk’s.
Regulators of Cell Cycle Progression
The activity of Cdk’s is regulated by four
mechanisms:
1. Association of Cdk’s and cyclin
partners.
Formation of specific Cdk/cyclin
complexes is controlled by cyclin
synthesis and degradation.
Figure 17.14 Mechanisms of Cdk regulation
Regulators of Cell Cycle Progression
2. Activation of Cdk/cyclin complexes
requires phosphorylation of threonine
at position 160.
This is catalyzed by CAK (Cdkactivating kinase), which is composed
of Cdk7/cyclin H.
Regulators of Cell Cycle Progression
3. Inhibitory phosphorylation of tyrosine
near the Cdk amino terminus,
catalyzed by Wee1 protein kinase.
The Cdk’s are then activated by
dephosphorylation by Cdc25 protein
phosphatases.
Regulators of Cell Cycle Progression
4. Binding of inhibitory proteins Cdk
inhibitors (CKIs).
In mammalian cells, there are two
families of Cdk inhibitors: Ink4 and
Cip/Kip
Table 17.1 Cdk Inhibitors
Regulators of Cell Cycle Progression
The combined effects of these modes of
Cdk regulation are responsible for
controlling cell cycle progression in
response to checkpoint controls and to
extracellular stimuli.
Regulators of Cell Cycle Progression
Proliferation of animal cells is regulated
by extracellular growth factors that
control progression through the
restriction point in late G1.
This implies that intracellular signaling
pathways ultimately act to regulate
components of the cell cycle
machinery.
Regulators of Cell Cycle Progression
D-type cyclins are one link between
growth factor signaling and cell cycle
progression.
Growth factors stimulate cyclin D1
synthesis through the Ras/Raf/MEK/ERK
pathway.
Cyclin D1 is synthesized as long as
growth factors are present.
Figure 17.15 Induction of D-type cyclins
Regulators of Cell Cycle Progression
Cyclin D1 is also rapidly degraded APC/C
ubiquitin ligase, so the intracellular
concentration falls rapidly if growth
factors are removed.
As long as growth factors are present
through G1, Cdk4,6/cyclin D complexes
drive cells through the restriction point.
Regulators of Cell Cycle Progression
Defects in cyclin D regulation could
contribute to the loss of growth
regulation characteristic of cancer cells.
Many human cancers arise as a result of
defects in cell cycle regulation.
Regulators of Cell Cycle Progression
Rb is a substrate protein of Cdk4,
6/cyclin D complexes, and is frequently
mutated in many human tumors.
It was first identified in retinoblastoma, a
rare inherited childhood eye tumor.
Rb is the prototype tumor suppressor
gene, a gene whose inactivation leads
to tumor development.
Regulators of Cell Cycle Progression
Proteins encoded by tumor suppressor
genes (including Rb and Ink4 Cdk
inhibitors) act as brakes that slow down
cell cycle progression.
Rb plays a key role in coupling cell cycle
machinery to the expression of genes
required for cell cycle progression.
Regulators of Cell Cycle Progression
In G0 or early G1, Rb binds to E2F
transcription factors, which suppresses
expression of genes involved in cell
cycle progression.
As cells pass through the restriction
point, Rb is phosphorylated by
Cdk4,6/cyclin D, and dissociates from
E2F, allowing transcription to proceed.
Figure 17.16 Cell cycle regulation of Rb and E2F
Regulators of Cell Cycle Progression
Progression through the restriction point
is mediated by activation of Cdk2/cyclin
E complexes.
In G0 and early G1, Cdk2/cyclin E is
inhibited by p27 (Cip/Kip family).
Figure 17.17 Activation of Cdk2/cyclin E
Regulators of Cell Cycle Progression
The inhibition of Cdk2 by p27 is relieved
by multiple mechanisms as cells
progress through G1.
• Growth factor signaling via
Ras/Raf/MEK/ERK and PI 3kinase/Akt pathways reduces
transcription and translation of p27.
Regulators of Cell Cycle Progression
• When Cdk2 becomes activated, it
phosphorylates p27 and targets it for
ubiquitylation.
• APC/C ubiquitin ligase is also
inhibited by Cdk2, so high levels of
cyclins are maintained through S and
G2.
Regulators of Cell Cycle Progression
Cdk2/cyclin E initiates S phase by
activating DNA synthesis at replication
origins.
Once a segment of DNA has been
replicated, control mechanisms prevent
reinitiation of DNA replication until the
cell cycle has been completed.
Regulators of Cell Cycle Progression
MCM helicase and origin recognition
complex (ORC) proteins bind to
replication origins during G1.
Cdk2/cyclin E activates the complex by
phosphorylating activating proteins.
Inhibition of APC/C leads to activation of
protein kinase DDK, which
phosphorylates MCM proteins directly.
Figure 17.18 Initiation of DNA replication
Regulators of Cell Cycle Progression
The high activity of Cdk’s during S, G2,
and M phases prevents MCM proteins
from reassociating with replication
origins.
Pre-replication complexes can only reform during G1, when Cdk activity is
low.
Regulators of Cell Cycle Progression
DNA damage checkpoints
Cell cycle arrest at DNA damage
checkpoints is mediated by protein
kinases ATM and ATR.
They then activate a signaling pathway
that leads to cell cycle arrest, DNA
repair, and sometimes, programmed
cell death.
Regulators of Cell Cycle Progression
ATM recognizes double-strand breaks;
ATR recognizes single-stranded or
unreplicated DNA.
They phosphorylate and activate the
checkpoint kinases Chk1 and Chk2.
Figure 17.19 Cell cycle arrest at the DNA damage checkpoints
Regulators of Cell Cycle Progression
Chk1 and Chk2 phosphorylate and
inhibit Cdc25 phosphatases, which are
required to activate Cdk1 and Cdk2.
Inhibition of Cdk2 results in cell cycle
arrest in G1 and S.
Inhibition of Cdk1 results in arrest in G2.
Regulators of Cell Cycle Progression
In mammalian cells, arrest is also
mediated by protein p53, which is
phosphorylated by both ATM and
Chk2.
p53 is a transcription factor; increased
levels lead to induction of Cdk inhibitor
p21.
p21 inhibits Cdk2/cyclin E or A
complexes, leading to cell cycle arrest.
Figure 17.20 Role of p53 in cell cycle arrest
Regulators of Cell Cycle Progression
The p53 gene is frequently mutated in
human cancers.
Loss of p53 prevents cell cycle arrest in
response to DNA damage, so the
damaged DNA is replicated and
passed on to daughter cells.
The Events of M Phase
M phase involves a major reorganization
of all cell components:
Chromosomes condense, nuclear
envelope breaks down, cytoskeleton
reorganizes to form the mitotic spindle,
and chromosomes move to opposite
poles.
Cell division (cytokinesis) usually
follows.
The Events of M Phase
Mitosis is divided into four stages:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Figure 17.21 Stages of mitosis in an animal cell
The Events of M Phase
Prophase—appearance of condensed
chromosomes (two sister chromatids).
The chromatids are attached at the
centromere, where proteins bind to
form the kinetochore (site of eventual
spindle attachment).
The Events of M Phase
The centrosomes (which duplicated
during interphase) separate and move
to opposite sides of the nucleus.
They serve as the two poles of the
mitotic spindle, which begins to form
during late prophase.
The Events of M Phase
In higher eukaryotes, prophase ends
when the nuclear envelope breaks
down (open mitosis).
In yeasts the nuclear envelope remains
intact (closed mitosis).
Spindle pole bodies are embedded in
the nuclear envelope; the nucleus
divides after migration of daughter
chromosomes.
Figure 17.23 Closed and open mitosis (Part 1)
Figure 17.23 Closed and open mitosis (Part 2)
Figure 17.22 Fluorescence micrographs of chromatin, keratin, and microtubules during mitosis in
newt lung cells
The Events of M Phase
Prometaphase—transition between
prophase and metaphase.
Spindle microtubules attach to
kinetochores.
The chromosomes shuffle back and forth
until they align on the metaphase plate.
The cell is then at metaphase.
Figure 17.22 Fluorescence micrographs of chromatin, keratin, and microtubules during mitosis in
newt lung cells
The Events of M Phase
Most cells are in metaphase only briefly
before proceeding to anaphase:
Links between sister chromatids break;
they separate and move to opposite
poles of the spindle.
Figure 17.22 Fluorescence micrographs of chromatin, keratin, and microtubules during mitosis in
newt lung cells
The Events of M Phase
Telophase: nuclei re-form and
chromosomes decondense.
Cytokinesis usually begins during late
anaphase and is almost complete by
the end of telophase.
Figure 17.22 Fluorescence micrographs of chromatin, keratin, and microtubules during mitosis in
newt lung cells
The Events of M Phase
Cdk1/cyclin B protein kinase (MPF) is
the master regulator of M phase
transition.
It activates other mitotic protein kinases
and directly phosphorylates structural
proteins involved in cellular
reorganization.
The Events of M Phase
Cdk1, Aurora and Polo-like kinases
are activated in a positive feedback
loop to signal entry into M phase.
Cdk1 activates Aurora kinases, which
activate Polo-like kinases, which in turn
activate Cdk1.
All of these protein kinases have multiple
roles in mitosis.
Figure 17.24 Mitotic protein kinases
The Events of M Phase
Condensation of chromatin by nearly a
thousandfold is a key event in mitosis.
Transcription ceases during
condensation.
The mechanism of condensation is not
fully understood, but it is driven by
condensins, “structural maintenance
of chromatin” (SMC) proteins.
The Events of M Phase
Condensins and cohesins contribute to
chromosome segregation.
Cohesins bind to DNA in S phase and
maintain links between sister chromatids.
Condensins are activated by Cdk1/cyclin
B phosphorylation; they replace the
cohesins, leaving sister chromatids
linked only at the centromere.
Figure 17.25 The action of cohesins and condensins
The Events of M Phase
Breakdown of the nuclear envelope
involves changes in all components:
• Nuclear membranes fragment
• Nuclear pore complexes dissociate
• Nuclear lamina depolymerizes—due to
phosphorylation of lamins by
Cdk1/cyclin B
Figure 17.26 Breakdown of the nuclear envelope
The Events of M Phase
The Golgi apparatus fragments into
vesicles, which are absorbed into the
ER or distributed to daughter cells at
cytokinesis.
Golgi breakdown is mediated by
phosphorylation of proteins by Cdk1
and Polo-like kinases.
The Events of M Phase
Reorganization of the cytoskeleton and
formation of the mitotic spindle
results from the dynamic instability of
microtubules.
Centrosome maturation and spindle
assembly are driven by Aurora and
Polo-like kinases at the centrosomes.
The Events of M Phase
Microtubule turnover rate increases,
resulting in depolymerization and
shrinkage of the interphase
microtubules.
The number of microtubules radiating
from the centrosomes also increases.
The Events of M Phase
Breakdown of the nuclear envelope
allows spindle microtubules to attach to
chromosomes at the kinetochores.
Chromosomes in prometaphase shuffle
back and forth due to activity of
microtubule motors at the kinetochore
and centrosomes.
Figure 17.27 Electron micrograph of microtubules attached to the kinetochore of a chromosome
The Events of M Phase
The balance of forces acting on the
chromosomes leads to their alignment
on the metaphase plate.
The spindle consists of kinetochore and
chromosomal microtubules, plus polar
microtubules which overlap in the center
of the cell, plus astral microtubules.
Figure 17.28 The metaphase spindle (Part 1)
Figure 17.28 The metaphase spindle (Part 2)
The Events of M Phase
At the spindle assembly checkpoint,
progression to anaphase is mediated
by activation of APC/C ubiquitin ligase
which is phosphorylated by
Cdk1/cyclin B.
The presence of even one unaligned
chromosome is sufficient to prevent
activation of the APC/C.
The Events of M Phase
Unattached kinetochores lead to the
assembly of the mitotic checkpoint
complex (MCC), which inhibits APC/C.
Once all chromosomes are aligned on
the spindle, the inhibitory complex is no
longer formed and APC/C is activated.
Figure 17.29 The spindle assembly checkpoint (Part 1)
The Events of M Phase
APC/C ubiquitylates cyclin B and
securin, which inactivates Cdk1 and
separase.
Separase degrades cohesin, which
breaks the link between sister
chromatids, allowing them to segregate
and move to opposite spindle poles.
Figure 17.29 The spindle assembly checkpoint (Part 2)
The Events of M Phase
Separation of chromosomes during
anaphase then proceeds by the action
of motor proteins associated with the
spindle microtubules.
APC/C also triggers degradation of
Aurora and Polo-like kinases, allowing
the cell to exit mitosis and return to
interphase.
Figure 17.30 A whitefish cell at anaphase
The Events of M Phase
Cytokinesis usually starts shortly after
anaphase starts and is triggered by
inactivation of Cdk1.
Cytokinesis of yeast and animal cells is
mediated by a contractile ring of actin
and myosin II filaments that forms
beneath the plasma membrane.
Figure 17.31 Cytokinesis of animal cells
The Events of M Phase
Ring formation is activated by Aurora
and Polo-like kinases.
The cell is cleaved in a plane that
passes through the metaphase plate.
Contraction of the actin-myosin filaments
pulls the plasma membrane inward,
eventually pinching the cell in half.
The Events of M Phase
In plant cells, cytokinesis proceeds by
formation of new cell walls and plasma
membranes.
In early telophase, vesicles carrying cell
wall precursors from the Golgi
accumulate at the former site of the
metaphase plate.
The Events of M Phase
The vesicles fuse to form a membraneenclosed disk, and polysaccharides
form the matrix of a new cell wall (cell
plate).
Plasmodesmata between the daughter
cells are formed as a result of
incomplete vesicle fusion.
Figure 17.32 Cytokinesis in higher plants
Meiosis and Fertilization
Meiosis is a specialized cell cycle that
reduces the chromosome number by
half, resulting in haploid daughter cells.
Unicellular eukaryotes can undergo
meiosis as well as reproduce by
mitosis.
In multicellular plants and animals,
meiosis is restricted to the germ cells.
Meiosis and Fertilization
Meiosis results in haploid progeny, each
with only one member of the pair of
homologous chromosomes that were
present in the diploid parent cell.
Two rounds of nuclear and cell division
(meiosis I and meiosis II) follow a
single round of DNA replication.
Figure 17.33 Comparison of meiosis and mitosis
Meiosis and Fertilization
In meiosis I, homologous chromosomes
pair with one another and then
segregate to different daughter cells.
Sister chromatids remain together, so
the daughter cells contain a single
member of each chromosome pair
(two sister chromatids).
Meiosis and Fertilization
Meiosis II resembles mitosis in that the
sister chromatids separate and
segregate to different daughter cells.
The result is four haploid daughter cells;
each has only one copy of each
chromosome.
Meiosis and Fertilization
Recombination between homologous
chromosomes occurs during prophase
of meiosis I.
Prophase I has five stages, based on
chromosome morphology: leptotene,
zygotene, pachytene, diplotene, and
diakinesis
Figure 17.34 Stages of the prophase of meiosis I
Meiosis and Fertilization
Recombination occurs at high rates
during meiosis.
In leptotene, the highly conserved
endonuclease Spo11 induces doublestrand breaks.
These lead to single-strand regions that
invade a homologous chromosome by
complementary base pairing.
Meiosis and Fertilization
Close association of homologous
chromosomes (synapsis) begins
during zygotene.
The zipperlike synaptonemal complex
forms along the length of the paired
chromosomes.
This keeps homologous chromosomes
closely associated and aligned.
Meiosis and Fertilization
Recombination is complete by the end of
pachytene, leaving the chromosomes
linked at sites of crossing over
(chiasmata).
The synaptonemal complex disappears at
diplotene, except at the chiasmata.
Each chromosome pair (a bivalent)
consists of four chromatids.
Meiosis and Fertilization
Diakinesis is the transition to
metaphase, during which the
chromosomes become fully
condensed.
Meiosis and Fertilization
Metaphase I: bivalent chromosomes
align on the spindle.
Kinetochores of sister chromatids are
oriented in the same direction
Kinetochores of homologous
chromosomes are pointed toward
opposite spindle poles.
Meiosis and Fertilization
Microtubules from the same pole of the
spindle attach to sister chromatids,
while microtubules from opposite poles
attach to homologous chromosomes.
Anaphase I: the chiasmata are disrupted
and homologous chromosomes
separate; sister chromatids remain
attached.
Figure 17.35 Chromosome segregation in meiosis I
Meiosis and Fertilization
Meiosis II starts immediately after
cytokinesis, usually before the
chromosomes have fully decondensed.
Meiosis II resembles mitosis.
Cytokinesis then follows, giving rise to
haploid daughter cells.
Meiosis and Fertilization
Vertebrate oocytes are useful models in
cell cycle research because they are
large and easy to manipulate in the
laboratory.
Meiosis of frog oocytes is regulated at
two unique points in the cell cycle.
Meiosis and Fertilization
Oocytes can remain arrested in the
diplotene stage of meiosis I for long
periods—up to 50 years in humans.
During this arrest, chromosomes
decondense and are actively
transcribed.
Oocytes grow very large and stockpile
materials for early embryonic growth.
Meiosis and Fertilization
In some animals, oocytes remain
arrested at diplotene until they are
fertilized.
Oocytes of most vertebrates resume
meiosis in response to hormonal
stimulation and proceed through
meiosis I prior to fertilization.
Meiosis and Fertilization
Cell division after meiosis I is asymmetric,
resulting in a small polar body and an
oocyte that retains its large size.
The oocyte enters meiosis II without
having re-formed a nucleus or
decondensed its chromosomes.
Most vertebrate oocytes are arrested
again at metaphase II, until fertilization.
Figure 17.36 Meiosis of vertebrate oocytes
Meiosis and Fertilization
Meiosis of oocytes is controlled by
Cdk1/cyclin B complexes:
• Hormonal stimulation activates
Cdk1/cyclin B, resulting in
progression to metaphase I.
• Levels of Cdk1/cyclin B determine
progression to the next stages.
Figure 17.37 Activity of Cdk1/cyclin B during oocyte meiosis
Meiosis and Fertilization
The factor responsible for metaphase II
arrest was identified in 1971, in the
same experiments that led to discovery
of MPF.
Cytoplasm from an egg arrested at
metaphase II was injected into an early
embryo cell, causing it to arrest at
metaphase.
Figure 17.38 Identification of cytostatic factor
Meiosis and Fertilization
Because this factor arrests mitosis, it is
called cytostatic factor (CSF).
Mos, a serine/threonine kinase, is an
essential component of CSF.
Mos is synthesized in oocytes at
completion of meiosis I and is required
for maintenance of Cdk1/cyclin B
activity.
Meiosis and Fertilization
The action of Mos results from activation
of ERK MAP kinase, but ERK plays a
different role in oocytes.
It activates another protein kinase, Rsk,
which maintains activity of MPF by
inhibiting cyclin B degradation.
Figure 17.39 Maintenance of Cdk1/cyclin B activity by the Mos protein kinase
Meiosis and Fertilization
Inhibition of cyclin B degradation is
mediated by inhibition of APC/C by
Emi2/Erp1, which is phosphorylated by
Rsk and inhibits APC/C via interaction
with Cdc20.
Oocytes can remain arrested at this
point for several days, awaiting
fertilization.
Meiosis and Fertilization
Fertilization:
The sperm binds to a receptor on the
egg surface and fuses with the egg
plasma membrane.
Fertilization mixes paternal and maternal
chromosomes and induces changes in
the egg cytoplasm important for further
development.
Figure 17.40 Fertilization
Meiosis and Fertilization
Binding of a sperm to its receptor signals
an increase in Ca2+ levels in the egg
cytoplasm, probably from hydrolysis of
PIP2.
Secretory vesicles release materials that
coat the egg and block entry of
additional sperm. This ensures a
normal diploid embryo.
Meiosis and Fertilization
Increased Ca2+ also signals completion
of meiosis.
Asymmetric cytokinesis gives rise to a
second small polar body.
After completion of meiosis, the fertilized
egg (zygote) contains two haploid
nuclei (pronuclei), one derived from
each parent.
Meiosis and Fertilization
The pronuclei replicate their DNA as
they migrate toward each other.
As they meet, the zygote enters M
phase of the first mitotic division.
Chromosomes align on one spindle.
Completion of mitosis gives rise to two
embryonic cells, each containing a new
diploid genome.
Figure 17.41 Fertilization and completion of meiosis