Mitosis and the Cell Cycle 10/21/05

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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

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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 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)
Sisterchromatids
Sister chromosomes separate during mitosis
Sister chromatids
Figure 12.4
Centromeres
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Overview of Mitosis
Prometaphase Nuclear envelope breaks down.
Chromosomes attach to spindle
DNA replication during Interphase
Prophase Chromosomes begin to condense. Spindle
starts to form
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Overview of Mitosis
Telophase Complete set of chromosomes at each
pole
Metaphase Chromosomes align in center of cell
Anaphase Sister chromatids separate
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Balanced attachment of spindle fibers to both
chromatids aligns chromosomes in metaphase
tug of war
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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
Nonkinetechore microtubules from opposite poles
overlap and push against each other, elongating
the cell
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Spindle fibers shorten at the kinetochore
Mark
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Kinetochore
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Cytokinesis

• Animal cells divide by constriction

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

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

See cell-cycle game at http//nobelprize.org/medi
cine/educational/2001/cellcycle.html
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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
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The Cell Cycle ClockCyclins 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 performs a cell cycle function
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The Human Cell Cycle
1 hour
4 hours
10 hours
9 hours
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How does the cell cycle cycle?
Focus first on entry and exit from mitosis
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• 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

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The Cell Cycle According to Cyclin Abundance
Cyiclin-CDK activity can also be controlled by
inhibitors
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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
CAK (CDK Activating Kinase)
is a kinase is a phosphatase
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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
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Cyclin Dependent Kinase Inhibitors (CKIs)
CDK
Cyclin
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Cell Cycle Regulators and Cancer
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Anaphase promoting complex
Triggers Chromosome separation Breakdown of
cyclin to re-start the cycle Breakdown of
geminin (to again allow replication)