Chiasmata are formed during meiosis :

Options

A. before metaphase I
B. during metaphase II
C. during prophase Il
D. after metaphase

Answer

To determine when chiasmata are formed during meiosis, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Chiasmata**: Chiasmata are structures that form between homologous chromosomes during meiosis. They are the physical manifestations of crossing over, where segments of DNA are exchanged between non-sister chromatids.

2. **Identify the Process of Crossing Over**: Crossing over occurs during the prophase stage of meiosis, specifically in prophase I. This is when homologous chromosomes pair up and exchange genetic material.

3. **Locate the Stage of Meiosis**: Since crossing over occurs in prophase I, the chiasmata will also be formed during this stage. After crossing over, the points where the chromatids are connected due to this exchange are referred to as chiasmata.

4. **Determine the Timing Relative to Metaphase**: Since prophase I occurs before metaphase I in the sequence of meiosis, chiasmata must be formed before metaphase I starts.

5. **Evaluate the Options**:
- Option A: Before metaphase I - This is correct as chiasmata form during prophase I.
- Option B: During metaphase II - Incorrect, as chiasmata are not formed during this stage.
- Option C: During prophase II - Incorrect, as chiasmata are already formed by this time.
- Option D: After metaphase - Incorrect, as chiasmata are formed prior to this phase.

6. **Conclusion**: The correct answer is that chiasmata are formed before metaphase I.

### Final Answer:
Chiasmata are formed during meiosis **before metaphase I** (Option A).

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