Which of the following checkpoint is predominantly activated when the DNA has started forming replication fork ?

Options

A. `G_1//S1`
B. `G_2//M`
C. M
D. Both `G_2//M and M`

Answer

To solve the question "Which of the following checkpoint is predominantly activated when the DNA has started forming replication fork?", we need to understand the cell cycle and the role of different checkpoints.

### Step-by-Step Solution:

1. **Understand the Cell Cycle**: The cell cycle consists of several phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis). Each phase has specific functions and checkpoints that regulate the progression of the cycle.

2. **Identify the Role of the S Phase**: The S phase is specifically where DNA replication occurs. During this phase, the DNA strands are unwound and replication forks are formed to synthesize new DNA strands.

3. **Checkpoints in the Cell Cycle**:
- **G1 Checkpoint**: This checkpoint assesses whether the cell is ready to enter the S phase. It checks for DNA damage and ensures that the cell has adequate resources to proceed.
- **G2 Checkpoint**: This checkpoint occurs before the cell enters mitosis (M phase) and checks for DNA damage after replication.
- **M Checkpoint**: This checkpoint ensures that all chromosomes are properly aligned before the cell divides.

4. **Determine the Relevant Checkpoint for DNA Replication**: Since the question specifies the formation of the replication fork, we focus on the S phase. The G1 checkpoint is activated before the S phase, but the actual replication and formation of the replication fork occur during the S phase.

5. **Conclusion**: The checkpoint predominantly activated when the DNA has started forming the replication fork is the **G1/S checkpoint**. This checkpoint ensures that the cell is ready to replicate its DNA.

### Final Answer:
The correct answer is **G1/S checkpoint**.