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.
Cyclins activate CDKs, which are present in their inactive form in the cell.
Once activated, CDKs phosphorylate (add phosphate group) to specific target proteins to activate or inactivate them.
The proteins can either inhibit or promote cell replication.
G1 checkpoint
-
Cell size
-
Nutrients
-
Growth factors
-
DNA damage
If no, then G0 phase (resting phase); cells can permanently be in this stage as well
G2 checkpoint
-
DNA damage
-
DNA replication completeness
Either pause at G2 and try to fix, or apoptosis
Spindle checkpoint
-
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)