The Cell Cycle

• Chromosomes duplicated and segregated into two genetically identical cells

Phases Of The Cell Cycle

• S phase: DNA synthesis• M phase: nuclear and cytoplasmic division• G1, G2: gaps, monitor internal and external conditions

Morphological Changes During Cell Cycle

• In metaphase replicated chromosomes aligned at equator• At anaphase sister chromosomes move to opposite poles

Control Of Cell Cycle

• System triggers series of events in correct order

• Checkpoints: arrest cell cycle if previous events not completed or environment unfavorable

Cyclin-Dependent Kinases

• Cdks active when bound to cyclin• Cyclin-Cdk complexes promote passage through particular stages• Cyclins undergo cycles of synthesis and degradation

• Activating phosphorylation- requires activating kinase• Inhibitory phosphorylation- inhibitory kinase,

removed by phosphatase

Regulation Of Cdk Activity By Phosphorylation

Regulation Of Cdk Activity By Cdk Inhibitor Proteins

• Inhibit activity of cyclin-Cdk complex• Prevent association of Cdk with cyclin

Proteolysis of Cyclins

• Cyclin degradation by transfer of ubiquitin• Mediated by ubiquitin ligases acting at particular stages• APC/C degrades M cyclins

Initiation Of DNA Replication

• S-cyclin-Cdk phosphorylates ORC for DNA replication to begin

• Re-replication prevented by Cdc6 phosphorylation and inhibition by geminin

Early Events Of Mitosis

M-cyclin-Cdk phosphorylation of: • Nuclear lamins• Protein complex that mediates

chromosome condensation• Proteins that regulate microtubules

• Triggered by APC/C ubiquitin ligase• APC/C targets degradation of separase inhibitor,

resulting in cleavage of chromatid cohesion complex

Anaphase

Exit From Mitosis

• Chromosomes decondense, spindle disassembles, nuclear envelope reforms

• Requires degradation of M-cyclin by APC/C-mediated ubiquitylation

Control Of G1 Progression By Retinoblastoma Protein

• G1-cyclin-Cdk phosphorylates Rb, resulting in

release of E2F and transcription of S phase genes

Mitogen Stimulation

• Extracellular signals that stimulate G1- and G1/S-cyclin-Cdks

Checkpoint Mechanisms

– DNA replication checkpoint: blocks G2→M– DNA damage checkpoint: blocks G2→M

• Mediated by Cdc25 phosphatase

– DNA damage checkpoint: blocks G1→S

• Mediated by p53

• DNA damage activates p53, which turns on p21 Cdk inhibitor

DNA Damage Checkpoint by p53

Mechanics Of M Phase

• Chromosomes condense• Mitotic spindle forms

• Nuclear envelope breaks down• Chromosomes attach to spindle

microtubules

• Sister chromatids move to opposite poles

• Chromosomes aligned at equator

• Chromosomes at poles• Nuclear envelope reforms

• Contractile ring of actin and myosin pinches cell in two

• Three classes: kinetochore, overlap, astral• Participate in movement of chromosomes

Classes Of Spindle Microtubules

• Anaphase A: chromosome movement toward poles by kinetochore MT shortening

• Anaphase B: separation of poles by motor proteins and overlap MT elongation

Movement At Anaphase

Forces at Anaphase A

• Kinetochore microtubule depolymerization• Microtubule flux: prior to anaphase, tension generated

from balanced tubulin addition (+), removal (-)

• Pushing by plus end directed motors• Pulling by minus end directed motors

Motor Proteins In Anaphase B