GENETICS, CELL AND MOLECULAR BIOLOGY · CELL BIOLOGY
Which of the following statements are CORRECT about eukaryotic cell cycle? P CDKs can phosphorylate proteins in the absence of cyclins. Q CDKs can be inactivated by phosphorylation. R Degradation of cyclins is required for cell cycle progression. S CDKs are not involved in chromosome condensation.
P and R only
P and S only
P, Q and R only
Q and R only
ANSWER
Q and R only — CDKs can be inactivated by inhibitory phosphorylation, and cyclin degradation is required for cell cycle progression.
Official key: D · Official answer key printed with the GATE 2020 BT paper (IIT Delhi)
Statement P: A CDK monomer lacks the active conformation needed for catalysis; only cyclin binding repositions the T-loop and activates the kinase. So a CDK cannot phosphorylate substrates without a bound cyclin, and P is FALSE.
Statement Q: CDK activity is turned off by inhibitory phosphorylation — Wee1 kinase phosphorylates CDK1 at Thr14 and Tyr15, and this phosphorylation is only removed by Cdc25 phosphatase to allow mitotic entry. So CDKs can indeed be inactivated by phosphorylation, and Q is TRUE.
Statement R: Progression through the cell cycle (e.g., mitotic exit, the metaphase-to-anaphase transition) requires the APC/C to ubiquitinate cyclins such as cyclin B, targeting them for proteasomal degradation; without this degradation CDK activity stays high and the cycle cannot proceed. So R is TRUE.
Statement S: The CDK1-cyclin B complex (MPF) phosphorylates condensin subunits and histone H3, directly driving mitotic chromosome condensation. So CDKs ARE involved in condensation, and S is FALSE.
Only Q and R are correct statements, which corresponds to option (D) — matching the official answer key.
KEY IDEA
CDK activity is controlled both positively (cyclin binding) and negatively (inhibitory phosphorylation, e.g. Wee1-mediated Tyr15/Thr14 phosphorylation of CDK1), and progression past checkpoints (such as the metaphase-to-anaphase transition and mitotic exit) requires the APC/C-mediated ubiquitination and proteasomal degradation of cyclins.
COMMON MISTAKE
Assuming CDKs act only through activation (cyclin binding, activating dephosphorylation by Cdc25) and forgetting that phosphorylation can also switch a CDK off (Wee1 pathway), which leads to wrongly rejecting statement Q; also assuming chromosome condensation is driven only by condensin loading and not recognizing that CDK1-cyclin B phosphorylates condensin subunits and histone H3, which would wrongly validate statement S.
From the GATE 2020 BT paper. Every question of this paper · Download the full paper · All 15 Cell biology questions, with notes
