Unit 12

Question 1

Coordination between different events of cell cycle

Answer 1

• Acheived through checkpoints
• Prevent entry into next phase until events of previous phase have been completed

Question 2

3 Main Checkpoints

Answer 2

1.) G1 checkpoint: Regulates transition from G1-to-S phase. Moniters environment before allowing cell cycle progression
2.) G2 checkpoint that regulates transition from G2-to-M phase. Moniters DNA for proper replication & the environment
3.) An M-phase checkpoint called spindle assembly checkpoint (SAC). This checkpoint regulates transition between metaphase & anaphase. It moniters the chromosomes for proper alignment at metaphase plate

Question 3

DNA damage can arrest cell cycle in G1 phase

Answer 3

• DNA damage activates 2 kinases (ATM & ATR)
• These kinases phosphorylate downstream kinases (Chk1 & Chk2) that phosphorylate p53 & stabilise it.
• Phosphorylated p53 can then induce expression of several key proteins
  1.) p21 - this protein binds to & inhibits complexes of cyclin - Cdk. In G1 this prevents inactivation of Rb & the cell remains in G1
  2.) Puma - this protein inhibits Bcl2, a key protein required for inhibition of cell death. Thus, prolonged / irreperable DNA damage induces cell death