1 Mitosis and the Cell Cycle 10/21/05
Dec 14, 2015
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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 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
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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
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Mitosis and the Cell Cycle
• Genetic information is copied exactly into 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 separate during mitosis
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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
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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
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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
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
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Mark
Spindle fibers shorten at the kinetochore
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Kinetochore
Chromosomemovement
Microtubule Motorprotein
Chromosome
Kinetochore
Tubulin
subunits
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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)
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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
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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
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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
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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?
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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 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.
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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 entry and exit from mitosis
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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
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Cyiclin-CDK activity can also be controlled 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 late
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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
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Cyclin Dependent Kinase Inhibitors (CKIs)
Cyclin
CDKp21
Cyclin
CDK4
Cyclin
CDK
CDK4p16
p16
p21
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Cell Cycle Regulators and Cancer
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Triggers:Chromosome separationBreakdown of cyclin to re-start the cycleBreakdown of geminin(to again allow replication)
Anaphase promoting complex