At which of the following stages, the chromosomes appear single, thin and thread like?

Options

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

Explanation

To determine at which stage the chromosomes appear single, thin, and thread-like, we can analyze each of the options provided in the question.

### Step-by-Step Solution:

1. **Understanding the Stages of Prophase I**:
- Prophase I of meiosis consists of several stages: Leptotene, Zygotene, Pachytene, and Diplotene. Each stage has distinct characteristics regarding the appearance and organization of chromosomes.

2. **Leptotene Stage**:
- In the Leptotene stage, individual chromosomes start to condense and become visible. They appear as long, thin, and thread-like structures. This is the initial stage where the chromosomes are not yet paired or homologous.

3. **Zygotene Stage**:
- During the Zygotene stage, homologous chromosomes begin to pair up (synapsis). At this point, the chromosomes are still visible but are starting to become thicker and more organized due to pairing.

4. **Pachytene Stage**:
- In the Pachytene stage, the chromosomes are even more condensed, and they appear thicker. This is the stage where crossing over occurs, and the chromosomes are not thread-like anymore.

5. **Diplotene Stage**:
- In the Diplotene stage, the paired homologous chromosomes begin to separate but remain connected at chiasmata. The chromosomes are still visible but are not in the thin, thread-like form.

6. **Conclusion**:
- Based on the characteristics of each stage, the chromosomes appear single, thin, and thread-like specifically during the **Leptotene stage**.

### Final Answer:
The chromosomes appear single, thin, and thread-like at the **Leptotene stage**.

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