The exchange of chromosomal segments between non-sister

Options

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

Answer

To solve the question regarding the exchange of chromosomal segments between non-sister chromatids, we will analyze the stages of meiosis, particularly focusing on the prophase I stages.

### 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 successive divisions: meiosis I and meiosis II.

2. **Phases of Meiosis I**: The first meiotic division (meiosis I) includes several stages, particularly during prophase I, which can be divided into five phases: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

3. **Leptotene Phase**: In this initial phase, chromosomes begin to condense and become visible under a microscope. However, there is no exchange of chromosomal segments occurring at this stage.

4. **Zygotene Phase**: During this phase, homologous chromosomes begin to pair up in a process called synapsis, forming a structure known as the synaptonemal complex. While pairing occurs, the exchange of chromosomal segments has not yet taken place.

5. **Pachytene Phase**: This is the critical phase for the exchange of chromosomal segments. During Pachytene, recombination nodules form at points where crossing over occurs between non-sister chromatids of homologous chromosomes. This is where the actual exchange of genetic material happens.

6. **Diplotene Phase**: In this phase, the synaptonemal complex begins to dissolve, and the homologous chromosomes start to separate, but the chiasmata (the points where crossing over occurred) remain visible. While crossing over has already happened in the previous phase, no new exchanges occur here.

7. **Diakinesis Phase**: This is the final stage of prophase I, where chromosomes condense further, and the nuclear envelope breaks down. There is no exchange of chromosomal segments occurring at this stage.

8. **Conclusion**: The exchange of chromosomal segments between non-sister chromatids occurs specifically during the Pachytene phase of prophase I in meiosis.

### Final Answer:
The correct answer is **Pachytene**.