Crossing over takes place between which chromatids and in which stage of the cell cycle ?

Options

A. Non-sister chromatids of non-homologous chromosomes at Zygotene stage of prophase I
B. Non-sister chromatids of homologous chromosomes at Pachytene stage of prophase I
C. Non-sister chromatids of homologous chromosomes at Zygotene stage of prophase I
D. Non-sister chromatids of non-homologous chromosomes at Pachytene stage of prophase I

Answer

**Step-by-Step Solution:**

1. **Understanding Crossing Over**:
- Crossing over is a genetic process that occurs during meiosis, specifically during prophase I. It involves the exchange of genetic material between chromatids.

2. **Identifying Chromatids Involved**:
- The chromatids involved in crossing over are non-sister chromatids. Non-sister chromatids are those that belong to homologous chromosomes, meaning they are from different parental origins but carry the same genes.

3. **Stages of Meiosis**:
- Meiosis consists of several stages, and prophase I is divided into several sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

4. **Finding the Correct Stage**:
- Crossing over specifically occurs during the pachytene stage of prophase I. During this stage, homologous chromosomes are closely paired, and the exchange of genetic material occurs.

5. **Conclusion**:
- Therefore, crossing over takes place between non-sister chromatids of homologous chromosomes during the pachytene stage of prophase I of meiosis.

**Final Answer**: Crossing over takes place between non-sister chromatids of homologous chromosomes at the pachytene stage of prophase I.

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