Transition from G2 to M phase of the ceil cycle is controlled by
**Core Concept**
The transition from the Gap 2 (G2) phase to the Mitosis (M) phase of the cell cycle is a critical regulatory step that ensures proper cell division and prevents uncontrolled cell growth. This process is controlled by a complex interplay of molecular mechanisms, including cyclin-dependent kinases (CDKs) and their inhibitors.
**Why the Correct Answer is Right**
The correct answer is **Cyclin B**. During the G2 phase, the cyclin B-CDK1 complex accumulates and becomes active, promoting the transition to the M phase. Cyclin B binds to CDK1, forming a complex that phosphorylates and inactivates the Wee1 kinase, which normally inhibits CDK1. This leads to the activation of CDK1, which in turn drives the cell into mitosis. The cyclin B-CDK1 complex also phosphorylates and inactivates the Cdc25C phosphatase, which dephosphorylates and activates CDK1.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because **p53** is a tumor suppressor protein that regulates the G1-S checkpoint, not the G2-M transition.
* **Option B:** This option is incorrect because **p21** is a cyclin-dependent kinase inhibitor that regulates the G1-S transition, not the G2-M transition.
* **Option D:** This option is incorrect because **Cyclin A** is involved in the S phase and G2 phase, but it is not the primary regulator of the G2-M transition.
**Clinical Pearl / High-Yield Fact**
The cyclin B-CDK1 complex is a key regulator of the G2-M transition, and its dysregulation can lead to cancer and other diseases. The use of cyclin B-CDK1 inhibitors is a promising area of research for cancer therapy.
**Correct Answer:** C. Cyclin B.