1

Mitosis and the Cell Cycle10/21/05

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)

G2Cyto

kines

is

Mito

sis

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 chromosomeHas two sister chromatids, which separate

during cell division

0.5 µm

Centromere

Sisterchromatids

Centromeres Sister chromatidsFigure 12.4

One chromosome, one DNA molecule

Duplication

One chromosome, two DNA molecules(Two attached chromatids)

Sister chromosomes separate during mitosis

6

G2 OF INTERPHASE

PROPHASE PROMETAPHASE

Centrosomes(with centriole pairs) Chromatin

(duplicated)

Early mitoticspindle

Aster

CentromereFragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore microtubule Figure 12.6

Nonkinetochoremicrotubules

Overview of Mitosis

DNA replication during Interphase Prophase:

Chromosomes begin to condense. Spindle starts to form

Prometaphase:Nuclear envelope breaks down. Chromosomes attach to spindle

7

Centrosome at one spindle pole

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate Nucleolus

forming

Cleavagefurrow

Nuclear envelopeforming

Figure 12.6

Overview of Mitosis

Metaphase:Chromosomes align incenter of cell

Anaphase:Sister chromatids separate

Telophase:Complete set of chromosomes at each pole

8

Balanced attachment of spindle fibers to both chromatidsaligns chromosomes in metaphase

“tug of war”

9

CentrosomeAster

Sisterchromatids

MetaphasePlate

Kinetochores

Overlappingnonkinetochoremicrotubules

Kinetochores microtubules

Centrosome

ChromosomesMicrotubules0.5 µm

1 µm

Figure 12.7

Nonkinetechore microtubules from opposite poles overlap and push against each other, elongating the cell

Kinetochore microtubules attach to centromeres and direct the poleward movement of chromosomes

Both chromatids must be captured by spindle fibers. If any kinetochores remain unattached, chromosomes will not separate

10

Mark

Spindle fibers shorten at the kinetochore

11

Kinetochore

Chromosomemovement

Microtubule Motorprotein

Chromosome

Kinetochore

Tubulin

subunits

12

CytokinesisAnimal cells divide by

constriction

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm

(a) Cleavage of an animal cell (SEM)Figure 12.9 A

Daughter cells

1 µmVesiclesforming cell plate

Wall of patent cellCell plateNew cell wall

(b) Cell plate formation in a plant cell (SEM)Figure 12.9 B

Plant cells build a partition (cell plate)

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)

G2Cyto

kines

is

Mito

sis

MITOTIC(M) PHASE

Figure 12.5

15

The clock has specific checkpoints: the cell cycle stops until a go-ahead signal is received

G1 checkpoint

G1G1

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.

See cell-cycle game at:http://nobelprize.org/medicine/educational/2001/cellcycle.html

16

Cell Cycle Control System

Figure 12.14

Control system

G1 checkpoint

G1

S

G2M

•MITOSIS EXIT:–All chromosomes attached to spindles?

•S-PHASE ENTRY (G1/S)–Mitosis Complete? –Growth/ Protein Synthesis adequate?–No DNA Damage?

•MITOSIS ENTRY (G2/M)–Replication Complete?–Growth/ Protein Synthesis adequate?–No DNA Damage?

17

The Cell Cycle Clock:Cyclins and Cyclin-dependent kinases

Cyclins– 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 performsa cell cycle function

19

The Human Cell Cycle

~ 10 hours

~ 9 hours

~ 4 hours

~ 1 hour

20

How does the cell cycle cycle?

Focus first on entry and exit from mitosis

21

MPF triggers:–assembly of the mitotic spindle

–breakdown of the nuclear envelope

–condensation of the chromosomes

Cyclin component degraded in anaphase

Cyclin B synthesized in S phase; Combines with cdk1 to make active MPF

Cyclin-CDK controls the cell cycle

22

Cyiclin-CDK activity can also be controlled by inhibitors

The Cell Cycle According to Cyclin Abundance

23

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(CDKActivatingKinase)

25

Conformational Changes Associated with CDK Phosphorylation

The T-loop blockssubstrate access

Free CDK CDK + Cyclin T161 phosphorylation

Binding of cyclinmoves the T-loop

Phosporylation movesthe T-loop more

26

Cyclin Dependent Kinase Inhibitors (CKIs)

Cyclin

CDKp21

Cyclin

CDK4

Cyclin

CDK

CDK4p16

p16

p21

27

Cell Cycle Regulators and Cancer

28

Triggers:Chromosome separationBreakdown of cyclin to re-start the cycleBreakdown of geminin(to again allow replication)

Anaphase promoting complex