1. Mitosis and the Cell Cycle 10/21/05

2. 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

3. Phases of the Cell Cycle
The cell cycle consists of
The mitotic phase (M)
Interphase G1 S G2
INTERPHASE G1
S (DNA synthesis) G2
Cytokinesis Mitosis
MITOTIC (M) PHASE
Figure 12.5

4. Mitosis and the Cell Cycle
Genetic information is copied exactly into each daughter cell
See it in action

5. 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

6. Kinetochore microtubule
Overview of Mitosis
G2 OF INTERPHASE
PROPHASE
PROMETAPHASE
Centrosomes (with centriole pairs)
Chromatin (duplicated)
Early mitotic spindle
Aster
Centromere
Fragments of nuclear envelope
Kinetochore
Nucleolus
Nuclear envelope
Plasma membrane
Chromosome, consisting of two sister chromatids
Kinetochore microtubule
Figure 12.6
Nonkinetochore microtubules
Prometaphase:
Nuclear envelope breaks down. Chromosomes attach to spindle
DNA replication during Interphase
Prophase:
Chromosomes begin to condense. Spindle starts to form

7. TELOPHASE AND CYTOKINESIS
Overview of Mitosis
Centrosome at one spindle pole
Daughter chromosomes
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Spindle
Metaphase plate
Nucleolus forming
Cleavage furrow
Nuclear envelope forming
Figure 12.6
Telophase:
Complete set of chromosomes at each pole
Metaphase:
Chromosomes align in
center of cell
Anaphase:
Sister chromatids separate

8. “tug of war” Balanced attachment of spindle fibers to both chromatids
aligns chromosomes in metaphase
“tug of war”

9. Both chromatids must be captured by spindle fibers
Both chromatids must be captured by spindle fibers. If any kinetochores remain unattached, chromosomes will not separate
Kinetochore microtubules attach to centromeres and direct the poleward movement of chromosomes
Centrosome
Aster
Sister chromatids
Metaphase Plate
Kinetochores
Overlapping nonkinetochore microtubules
Kinetochores microtubules
Chromosomes
Microtubules
0.5 µm
1 µm
Figure 12.7
Nonkinetechore microtubules from opposite poles overlap and push against each other, elongating the cell

10. Spindle fibers shorten at the kinetochore
Mark

11. Kinetochore Chromosome movement Kinetochore Tubulin subunits
Microtubule
Motor protein
Chromosome
Kinetochore
Tubulin subunits

12. Cytokinesis Animal cells divide by constriction
Plant cells build a partition (cell plate)
Cleavage furrow
Contractile ring of microfilaments
Daughter cells
100 µm
(a) Cleavage of an animal cell (SEM)
Figure 12.9 A
Daughter cells
1 µm
Vesicles forming cell plate
Wall of patent cell
Cell plate
New cell wall
(b) Cell plate formation in a plant cell (SEM)
Figure 12.9 B

13. 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

14. Phases of the Cell Cycle
The cell cycle consists of
The mitotic phase (M)
Interphase G1 S G2
INTERPHASE G1
S (DNA synthesis) G2
Cytokinesis Mitosis
MITOTIC (M) PHASE
Figure 12.5

15. The clock has specific checkpoints: the cell cycle stops until a go-ahead signal is received
See cell-cycle game at:
G1 checkpoint G1 G0
(a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues      on in the cell cycle.
(b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a
nondividing state.
Figure A, B

16. Cell Cycle Control System
S-PHASE ENTRY (G1/S)
Mitosis Complete?
Growth/ Protein Synthesis adequate?
No DNA Damage?
G1 checkpoint
MITOSIS EXIT:
All chromosomes attached to spindles?
Control system S G1 G2 M
MITOSIS ENTRY (G2/M)
Replication Complete?
Growth/ Protein Synthesis adequate?
No DNA Damage?
Figure 12.14

17. The Cell Cycle Clock: Cyclins and Cyclin-dependent kinases
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)
Cyclin levels in the cell rise and fall with the stages of the cell cycle.
Cdk levels remain stable, but each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated.

18. Phosphorylation of CDK Targets Changes Their Activity
Now performs
a cell cycle function

19. The Human Cell Cycle
~ 1 hour
~ 4 hours
~ 10 hours
~ 9 hours

20. How does the cell cycle cycle?
Focus first on entry and exit from mitosis

21. 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

22. The Cell Cycle According to Cyclin Abundance
Cyiclin-CDK activity can also be controlled by inhibitors

23. Isolate mutants that divide too early or too late
How are CDKs Regulated?
Isolate mutants that divide too early or too late

24. CDKs are Regulated by Phosphorylation is a kinase is a phosphatase CAK
Activating
Kinase)
is a kinase is a phosphatase

25. Conformational Changes Associated with CDK Phosphorylation
Free CDK
CDK + Cyclin
T161 phosphorylation
The T-loop blocks
substrate access
Binding of cyclin
moves the T-loop
Phosporylation moves
the T-loop more

26. Cyclin Dependent Kinase Inhibitors (CKIs)
CDK
CDK
p21
Cyclin
p16
Cyclin
CDK4
CDK4
p16

27. Cell Cycle Regulators and Cancer

28. Anaphase promoting complex
Triggers:
Chromosome separation
Breakdown of cyclin to re-start the cycle
Breakdown of geminin
(to again allow replication)