Mitosis and the Cell Cycle
10/21/05
1
Lecture Outline
• Two goals of the Cell Cycle:
– Make one cell into two
– Must accurately replicate the genetic material
• Mitosis (replicate and distribute the
chromosomes)
– Major phases
– Mechanics of chromosome segregation
• Cytokinesis (how does one cell become two?)
• Replication of the cytoplasm and organelles
• Control of the Cell Cycle
– Cyclins and CDKs
– The importance of checkpoints for quality control
2
Phases of the Cell Cycle
• The cell cycle consists of
– The mitotic phase (M)
– Interphase
• G1
• S
• G2
INTERPHASE
G1
S
(DNA synthesis)
G2
Figure 12.5
3
Mitosis and the Cell Cycle
• Genetic information is copied exactly
into each daughter cell
• See it in action
4
• Each duplicated chromosome
Has two sister chromatids, which separate
during cell division
0.5 µm
One chromosome, one DNA molecule
Duplication
Centromere
One chromosome, two DNA molecules
(Two attached chromatids)
Sister
chromatids
Sister chromosomes separate
during mitosis
Figure 12.4
Centromeres
Sister chromatids
5
Overview of Mitosis
G2 OF
INTERPHASE
Centrosomes
Chromatin
(with centriole pairs)
(duplicated)
PROPHASE
Early mitotic
spindle
Aster
Centromere
PROMETAPHASE
Fragments
Kinetochore
of nuclear
envelope
Nonkinetochore
microtubules
Prometaphase:
Kinetochore
Chromosome, consisting
Nuclear
Plasma
Nucleolus
Nuclear envelope
Figure 12.6
microtubule
of two sister chromatids
envelope membrane
breaks down.
Chromosomes
DNA replication during
Prophase:
attach to spindle
Interphase
Chromosomes begin to condense.
6
Spindle starts to form
Overview of Mitosis
METAPHASE
ANAPHASE
Metaphase
plate
Figure 12.6
Spindle
Centrosome at Daughter
one spindle pole chromosomes
Metaphase:
Chromosomes align in
center of cell
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nuclear
envelope
forming
Anaphase:
Sister chromatids separate
Nucleolus
forming
Telophase:
Complete
set of
chromosom
es at each
pole
7
Balanced attachment of spindle fibers to both chromatids
aligns chromosomes in metaphase
“tug of war”
8
Kinetochore microtubules
attach to centromeres
and direct the poleward
movement of
chromosomes
Nonkinetechore
microtubules from opposite
poles overlap and push
Both chromatids must be captured by
spindle fibers. If any kinetochores
remain unattached, chromosomes will
not separate
Aster
Sister
chromatids
Centrosome
Metaphase
Plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Kinetochores
microtubules
Microtubules
0.5 µm
Chromosomes
against each other, elongating
the cell
Figure 12.7
Centrosome
1 µm
9
Spindle fibers shorten at the kinetochore
Mark
10
Kinetochore
Chromosome
movement
Microtubule
Kinetochore
Tubulin
subunits
Motor
protein
Chromosome
11
Cytokinesis
Animal cells divide by
constriction
Cleavage furrow
Contractile ring of
microfilaments
Figure 12.9 A
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
Plant cells build a
partition (cell plate)
Vesicles Wall of
1 µm
forming patent cellCell plate
New cell wall
cell plate
Daughter cells
(b) Cell
Figure 12.9
B plate formation in a plant cell
12 (SEM)
How do the cytoplasmic
organelles divide?
• Mitochondria (and chloroplasts) are
present in multiple copies, and
randomly segregate into the two
daughter cells.
• Membrane bound organelles (e.g.
ER) fragment along with the nuclear
membrane and are reconstructed in
the daughter cells
13
Phases of the Cell Cycle
• The cell cycle consists of
– The mitotic phase (M)
– Interphase
• G1
• S
• G2
INTERPHASE
G1
S
(DNA synthesis)
G2
Figure 12.5
14
The clock has specific checkpoints: the cell cycle
stops until a go-ahead signal is received
See cell-cycle game at:
http://nobelprize.org/medicine/educational/2001/cellcycle.html
G0
G1 checkpoint
G1
G1
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Cell Cycle Control System
•S-PHASE ENTRY (G1/S)
–Mitosis Complete?
–Growth/ Protein
Synthesis adequate?
–No DNA Damage?
•MITOSIS EXIT:
–All chromosomes
attached to
spindles?
•MITOSIS ENTRY (G2/M)
–Replication Complete?
–Growth/ Protein Synthesis
adequate?
–No DNA Damage?
G1 checkpoint
Control
system
S
G1
M
G2
Figure 12.14
16
The Cell Cycle Clock:
Cyclins and Cyclin-dependent kinases
Cyclins
Cyclin levels in the cell rise and fall
with the stages of the cell cycle.
– G1 cyclin (cyclin D)
– S-phase cyclins (cyclins E and A)
– M-phase cyclins (cyclins B and A)
Cyclin-dependent kinases (Cdks)
– G1 Cdk (Cdk4)
– S-phase Cdk ((Cdk2)
– M-phase Cdk (Cdk1)
Cdk levels remain stable, but each
must bind the appropriate cyclin
(whose levels fluctuate) in order to be
activated.
17
Phosphorylation of CDK Targets Changes Their Activity
Now performs
a cell cycle function
18
The Human Cell Cycle
~ 1 hour
~ 4 hours
~ 10 hours
~ 9 hours
19
How does the cell cycle cycle?
Focus first on entry
and exit from
mitosis
20
Cyclin-CDK controls
the cell cycle
Cyclin B synthesized in S phase;
Combines with cdk1 to make active
MPF
Cyclin component degraded
in anaphase
MPF triggers:
–assembly of the mitotic spindle
–breakdown of the nuclear envelope
–condensation of the chromosomes
21
The Cell Cycle According to Cyclin Abundance
Cyiclin-CDK activity can also be
controlled by inhibitors22
How are CDKs Regulated?
Isolate mutants that divide too early or too late
23
CDKs are Regulated
by Phosphorylation
is a kinase
is a phosphatase
CAK
(CDK
Activating
Kinase)
24
Conformational Changes Associated
with CDK Phosphorylation
Free CDK
The T-loop blocks
substrate access
CDK + Cyclin
Binding of cyclin
moves the T-loop
T161 phosphorylation
Phosporylation moves
25
the T-loop more
Cyclin Dependent Kinase Inhibitors (CKIs)
p21
CDK
CDK
Cyclin
Cyclin
p21
p16
CDK4
Cyclin
CDK4
p16
26
Cell Cycle Regulators and
Cancer
27
Anaphase promoting complex
Triggers:
Chromosome separation
Breakdown of cyclin to re-start the cycle
Breakdown of geminin
(to again allow replication)
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