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Mitosis and the Cell Cycle10/21/05

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Lecture Outline• Two goals of the Cell Cycle:

– Make one cell into two– Must accurately replicate the genetic material

• Mitosis (replicate and distribute thechromosomes)– 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

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Phases of the Cell Cycle• The cell cycle consists of

– The mitotic phase (M)– Interphase

• G1• S• G2

INTERPHASE

G1S

(DNA synthesis)

G2Cyto

kines

is

Mitosis

MITOTIC(M) PHASE

Figure 12.54

Mitosis and the Cell Cycle

• Genetic information is copied exactlyinto each daughter cell

• See it in action

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• 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 separateduring mitosis

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G2 OFINTERPHASE PROPHASE PROMETAPHASE

Centrosomes(with centriole pairs) Chromatin

(duplicated)Early mitoticspindle

AsterCentromere

Fragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore microtubule Figure 12.6

Nonkinetochoremicrotubules

Overview of Mitosis

DNA replication duringInterphase Prophase:

Chromosomes begin to condense.Spindle starts to form

Prometaphase:Nuclear envelopebreaks down.Chromosomesattach to spindle

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Centrosome at one spindle pole

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate Nucleolus

formingCleavagefurrow

Nuclear envelopeformingFigure 12.6

Overview of Mitosis

Metaphase:Chromosomes align incenter of cell

Anaphase:Sister chromatids separate

Telophase:Completeset ofchromosomes at eachpole

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Balanced attachment of spindle fibers to both chromatidsaligns chromosomes in metaphase

“tug of war”

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CentrosomeAster

Sisterchromatids

MetaphasePlate

Kinetochores

Overlappingnonkinetochoremicrotubules

Kinetochoresmicrotubules

Centrosome

ChromosomesMicrotubules 0.5 µm

1 µm

Figure 12.7

Nonkinetechoremicrotubules from oppositepoles overlap and pushagainst each other, elongatingthe cell

Kinetochore microtubulesattach to centromeresand direct the polewardmovement ofchromosomes

Both chromatids must be captured byspindle fibers. If any kinetochoresremain unattached, chromosomes willnot separate

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Mark

Spindle fibers shorten at the kinetochore

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Kinetochore

Chromosomemovement

Microtubule Motorprotein

Chromosome

Kinetochore

Tubulinsubunits

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CytokinesisAnimal cells divide by

constriction

Cleavage furrow

Contractile ring ofmicrofilaments 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 apartition (cell plate)

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How do the cytoplasmicorganelles divide?

• Mitochondria (and chloroplasts) arepresent in multiple copies, andrandomly segregate into the twodaughter cells.

• Membrane bound organelles (e.g.ER) fragment along with the nuclearmembrane and are reconstructed inthe daughter cells

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Phases of the Cell Cycle• The cell cycle consists of

– The mitotic phase (M)– Interphase

• G1• S• G2

INTERPHASE

G1S

(DNA synthesis)

G2Cyto

kines

is

Mitosis

MITOTIC(M) PHASE

Figure 12.5

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The clock has specific checkpoints: the cell cyclestops 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.

Figure 12.15 A, B

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

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Cell Cycle Control System

Figure 12.14

Control system

G1 checkpoint

G1

S

G2M

•MITOSIS EXIT:–All chromosomesattached tospindles?

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

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

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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 fallwith the stages of the cell cycle.

Cdk levels remain stable, but eachmust bind the appropriate cyclin(whose levels fluctuate) in order to beactivated.

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Phosphorylation of CDK Targets Changes Their Activity

Now performsa cell cycle function

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The Human Cell Cycle

~ 10 hours

~ 9 hours

~ 4 hours

~ 1 hour

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How does the cell cycle cycle?

Focus first on entryand exit frommitosis

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MPF triggers:–assembly of the mitotic spindle –breakdown of the nuclear envelope –condensation of the chromosomes

Cyclin component degradedin anaphase

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

Cyclin-CDK controlsthe cell cycle

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Cyiclin-CDK activity can also becontrolled by inhibitors

The Cell Cycle According to Cyclin Abundance

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How are CDKs Regulated?

Isolate mutants that divide too early or too late24

CDKs are Regulated by Phosphorylation

is a kinase is a phosphatase

CAK(CDKActivatingKinase)

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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)

CyclinCDK

p21

CyclinCDK4

CyclinCDK

CDK4p16

p16

p21

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Cell Cycle Regulators andCancer

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G1G2 Metaphase Anaphase

Cyclin-Cdk Cyclin-CdkSecurin

Anaphase promoting complex

APC

Cdc20 E2-Ub Cdh1

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

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29MCB Fig. 13-19 30

Prophase Prometaphase

Metaphase Anaphase Telophase