Question

---- cyclin dependent kinase (CDK) is involved in the progression past restriction point at G1/S boundary of cell A

• A. CDK1
• B. CDK2
• C. CDK3
• D. CDK4

Hint:

Associate CDK2 with the G1/S transition (Cyclin E-CDK2) and early S-phase (Cyclin A-CDK2).
This kinase is essential to pass the point of no return (the restriction point).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Cell cycle progression is strictly regulated by the sequential activation of Cyclin-Dependent Kinases (CDKs) by their partner cyclin proteins.

Step 2: Detailed Explanation:
Let us review the CDKs active at different cell cycle transitions:
1. CDK4 CDK6 (paired with Cyclin D) are active in early G1 phase and help initiate phosphorylation of the Retinoblastoma (Rb) protein.
2. CDK2 (paired with Cyclin E) is activated in late G1 phase.
This Cyclin E-CDK2 complex drives the hyperphosphorylation of Rb, releasing E2F transcription factors to allow progression past the crucial restriction point (R-point) at the G1/S boundary, initiating S-phase entry.
3. CDK1 (paired with Cyclin B) regulates mitosis entry.
Therefore, CDK2 is the kinase responsible for G1/S boundary restriction point progression.

Step 3: Final Answer:
The correct option is 2, which corresponds to CDK2.