Question

2. Cyclin B plus CDC2 are necessary for a checkpoint at the G2->M transition that makes sure chromosomes are intact before mitosis begins. If you irradiate cells in G2 phase with gamma rays, which damages the DNA that was successfully replicated in S-phase, cells delay entry into mitosis until the DNA damage is repaired. If you inactivated that Cyclin B/ CDC2 checkpoint with the drug wortmannin and irradiated cells in G2, what would happen? a) Cells would never enter mitosis b) Cells would delay entry into mitosis c) Cells would enter mitosis at the same rate as non-irradiated cells d) Cells would accelerate entry into mitosis e) None of the above

Answer 1

Answer is A. Cells would never enter mitosis. This is due to the fact that CDC2 is needed to dephosphorylate Tyrosine and Threonine residues phosphorylated by WEE1 kinase. Thus CDC2 acts a phosphatase and thus activates Cyclin B/Cdk1 complex which makes the cells enter mitosis. An inactivated CDC2 would therefore lose its phosphatase activity and cells would be arrested at G2 stage.