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 regarding which checkpoint is predominantly activated when the DNA has started forming the replication fork, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Cell Cycle Phases**:
- The cell cycle consists of several phases: G1 (first growth phase), S (synthesis phase), G2 (second growth phase), and M (mitosis).
- During the S phase, DNA replication occurs, and the amount of DNA in the cell doubles.

2. **Identifying the Checkpoints**:
- Checkpoints are regulatory mechanisms that ensure the proper progression of the cell cycle.
- The main checkpoints are:
- G1 checkpoint (G1/S checkpoint): This checkpoint assesses whether the cell is ready to enter the S phase.
- G2 checkpoint: This checkpoint checks for DNA damage and ensures that all DNA is replicated before entering mitosis.
- M checkpoint: This checkpoint ensures that all chromosomes are properly attached to the spindle before anaphase begins.

3. **Focus on the S Phase**:
- The question specifically asks about the checkpoint activated when DNA replication begins (i.e., when the replication fork is formed).
- The S phase is where DNA replication occurs, and the G1/S checkpoint is crucial in determining if the cell can proceed to this phase.

4. **Activation of the G1/S Checkpoint**:
- The G1/S checkpoint is activated in response to various signals, including DNA damage and the cell's size.
- When the DNA has started forming the replication fork, it indicates that the cell is preparing to enter the S phase, thus activating the G1/S checkpoint.

5. **Conclusion**:
- Therefore, the checkpoint predominantly activated when the DNA has started forming the replication fork is the **G1/S checkpoint**.

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

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