When are bivalents of chromosomes first clearly visible in meiosis ?

Options

A. Leptotene
B. Diplotene
C. Pachytene
D. Diakinesis

Answer

To answer the question "When are bivalents of chromosomes first clearly visible in meiosis?", we can break down the process of meiosis and analyze the stages involved.

### Step-by-Step Solution:

1. **Understanding Meiosis**:
- Meiosis is a type of cell division that reduces the chromosome number by half, resulting in four haploid cells. It consists of two main stages: Meiosis I and Meiosis II, with each stage further divided into phases.

2. **Identifying the Phases of Meiosis**:
- The first stage of Meiosis I is Prophase I, which is further divided into several sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

3. **Leptotene Stage**:
- In the Leptotene stage, chromosomes begin to condense and become visible, but they do not form bivalents yet. Therefore, bivalents are not visible in this stage.

4. **Zygotene Stage**:
- During Zygotene, homologous chromosomes begin to pair up, a process known as synapsis. However, the bivalents are still not clearly visible at this stage.

5. **Pachytene Stage**:
- In the Pachytene stage, the chromosomes are fully paired, and the bivalents (paired homologous chromosomes) become clearly visible. This is the stage where crossing over occurs, leading to genetic variation.

6. **Diplotene Stage**:
- In Diplotene, the homologous chromosomes start to separate slightly, but the bivalents are still visible. However, they were first clearly visible in the Pachytene stage.

7. **Diakinesis Stage**:
- In Diakinesis, the chromosomes condense further, and the bivalents are still visible, but they were first clearly visible in the Pachytene stage.

### Conclusion:
The bivalents of chromosomes are first clearly visible in the **Pachytene stage** of meiosis.

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