Question

Which statement about M-phase cyclin is correct

• A. Cyclin synthesis and destruction is essential for cell cycle progression
• B. Cyclin synthesis and no destruction is essential for cell cycle progression
• C. Cyclins play no role in cell cycle progression
• D. No Cyclin synthesis and no destruction is essential for cell cycle progression

Hint:

To enter mitosis, M-cyclin must be synthesized to activate Cdk1 (MPF).
To exit mitosis, M-cyclin must be completely destroyed by the proteasome to turn off Cdk1.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The eukaryotic cell cycle is a highly regulated, sequential process consisting of interphase (G1, S, G2 phases) and mitosis (M phase).
Progression through these phases is controlled by a biochemical engine consisting of two key classes of proteins:
1. Cyclin-Dependent Kinases (CDKs): Catalytically active enzymes whose levels remain constant throughout the cell cycle.
2. Cyclins: Regulatory proteins that bind and activate CDKs.
Unlike CDKs, the concentration of cyclins fluctuates (cycles) dramatically during the cell cycle, dictating when specific CDKs are active.

Step 2: Detailed Explanation:
M-phase cyclin (such as Cyclin B) regulates entry into and exit from mitosis.
Let us analyze why both the synthesis and destruction of this cyclin are critical:
1. Synthesis:
During the G2 phase, M-phase cyclin is continuously synthesized and accumulates inside the cell.
It binds to its partner CDK (Cdk1), forming the active Mitosis-Promoting Factor (MPF).
This active complex phosphorylates target proteins required for chromosomes to condense, the nuclear envelope to break down, and the spindle apparatus to form, triggering entry into mitosis.
2. Destruction:
To exit mitosis and complete cell division, MPF activity must be turned off.
This inactivation is achieved through the rapid, targeted destruction of M-phase cyclin.
At the metaphase-anaphase transition, the Anaphase-Promoting Complex/Cyclosome (APC/C) ubiquitinates the cyclin, targeting it for degradation by the $26\text{S}$ proteasome.
This degradation inactivates Cdk1, allowing chromosomes to decondense and the nuclear envelope to reform, completing cell division.
If cyclin is synthesized but not destroyed (Option B), the cell remains permanently locked in metaphase and cannot complete the cell cycle.
If cyclin is not synthesized (Option D), the cell cannot enter mitosis.
Therefore, both the timely synthesis and the rapid destruction of cyclins are essential for cell cycle progression.

Step 3: Final Answer:
Consequently, the correct statement is that cyclin synthesis and destruction is essential for cell cycle progression.