During a cell cycle the transition from `G_(1)` to S phase and from `G_(2)` to M phase requires the activation of which of the following?

Options

A. Inducer enzyme
B. ATP
C. Cyclins
D. Cyclin-CDK complex

Answer

To answer the question regarding the activation required for the transition from G1 to S phase and from G2 to M phase during the cell cycle, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Cell Cycle Phases**:
- The cell cycle consists of several phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis).
- The transitions between these phases are critical for proper cell division and function.

2. **Identifying Key Checkpoints**:
- There are specific checkpoints in the cell cycle where the cell assesses whether it is ready to proceed to the next phase.
- The first checkpoint occurs at the transition from G1 to S phase, and the second checkpoint occurs at the transition from G2 to M phase.

3. **Role of Cyclin-Dependent Kinases (CDKs)**:
- The progression through these checkpoints is regulated by proteins known as cyclin-dependent kinases (CDKs).
- CDKs are activated when they bind to specific regulatory proteins called cyclins.

4. **Activation of CDK Complex**:
- For the transitions from G1 to S and G2 to M, the activation of the CDK complex is essential.
- This activation allows the cell to move forward in the cell cycle, promoting the necessary biochemical processes for DNA replication and cell division.

5. **Selecting the Correct Answer**:
- The question asks which factor is required for these transitions. Among the options provided, the correct answer is the CDK complex (cyclin-dependent kinases complex).
- Therefore, the correct answer is option 4: the CDK complex.

### Final Answer:
The activation of the CDK complex (cyclin-dependent kinases) is required for the transition from G1 to S phase and from G2 to M phase during the cell cycle.

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