Q.No. 29 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

• (A) P and R only
• (B) P and S only
• (C) P, Q and R only

• • P: CDKs can phosphorylate proteins in the absence of cyclins – Incorrect. CDKs depend on cyclin binding for activation and substrate phosphorylation, as cyclins induce conformational changes enabling kinase activity.​

  • Q: CDKs can be inactivated by phosphorylation – Correct. Inhibitory phosphorylation on Thr14/Tyr15 by kinases like WEE1 blocks CDK activity, providing checkpoints such as during DNA damage.​

  • R: Degradation of cyclins is required for cell cycle progression – Correct. Ubiquitin-mediated cyclin degradation by the APC/C resets CDK activity, enabling phase transitions like mitosis exit.​

  • S: CDKs are not involved in chromosome condensation – Incorrect. CDK1-cyclin B phosphorylates condensins and histone H3, directly driving chromosome condensation in prophase.​(D) Q and R only

    Eukaryotic Cell Cycle Regulation: CDKs, Cyclins, and Key Mechanisms

    CDKs require cyclins for activity, can be inactivated by phosphorylation, and cyclin degradation drives progression, while CDKs promote chromosome condensation. Only statements Q and R are correct. The answer is (D) Q and R only.​

    Statement Analysis

  The eukaryotic cell cycle relies on precise regulation by cyclin-dependent kinases (CDKs) and cyclins to ensure DNA replication and division occur accurately. Key processes like CDK phosphorylation and cyclin degradation act as molecular switches, preventing errors like cancer. This guide analyzes common exam statements on eukaryotic cell cycle mechanisms, ideal for competitive exams like IIT JAM.

  CDK-Cyclin Dependency

  CDKs remain inactive without cyclins, which bind to activate their kinase function across G1, S, G2, and M phases. For instance, cyclin B-CDK1 triggers mitosis entry. Without cyclins, no substrate phosphorylation occurs, refuting claims of cyclin-independent activity.​

  Inactivation by Phosphorylation

  CDKs can be inactivated by phosphorylation on specific residues (Thr14/Y15), enforced by WEE1 kinase during checkpoints. This halts progression until DNA repair, with CDC25 reversing it. Such dual regulation—activating (T-loop) and inhibitory—fine-tunes timing.​

  Role of Cyclin Degradation

  Degradation of cyclins via APC/C ubiquitin ligase is essential for progression. Cyclin B destruction inactivates CDK1, allowing mitotic exit and cytokinesis. Periodic synthesis-degradation cycles ensure unidirectional advancement.​

  Chromosome Condensation

  CDKs drive chromosome condensation by phosphorylating condensins and lamina proteins, compacting chromatin for segregation. This counters incorrect statements denying CDK involvement.​

  Statement	Correct?	Mechanism
  P	No	Cyclins required for CDK activation ​
  Q	Yes	Inhibitory phosphorylation by WEE1 ​
  R	Yes	APC/C-mediated ubiquitin degradation ​
  S	No	CDK1 phosphorylates condensins ​

  Master these eukaryotic cell cycle facts for exams—focus on Q and R as correct in multiple-choice contexts.