CELL CYCLE CONTROL
First checkpoint: Between G1 and S phase (before DNA replication)
Second checkpoint: Between G2 and mitosis
Cyclin dependent kinases (CDKs) + cyclins are used to control cell cycle.
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Cyclins activate CDKs, which are present in their inactive form in the cell.
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Once activated, CDKs phosphorylate (add phosphate group) to specific target proteins to activate or inactivate them.
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The proteins can either inhibit or promote cell replication.
G1 checkpoint
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Cell size
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Nutrients
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Growth factors
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DNA damage
If no, then G0 phase (resting phase); cells can permanently be in this stage as well
G2 checkpoint
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DNA damage
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DNA replication completeness
Either pause at G2 and try to fix, or apoptosis
Spindle checkpoint
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Chromosome attachment to spindle at metaphase plate
Pause until fixed
S CDK2-A
G1 CDK2-E
G1 CDK4-D
G2 CDK1-B
(PROMOTE DNA REPLICATION)
(INHIBIT Rb FROM STOPPING DNA REPLICATION)
(PROMOTE MITOSIS)