Chromosome syndesis or bivalent formation occurs in

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the phase in which chromosome synapsis or bivalent formation occurs. This is a process related to meiosis, which is a type of cell division.

2. **Identifying Meiosis Stages**: Meiosis consists of two main stages: Meiosis I and Meiosis II. The focus here is on Meiosis I, specifically during Prophase I.

3. **Breaking Down Prophase I**: Prophase I of meiosis is divided into five sub-stages:
- **Leptotene**: Chromosomes start to condense.
- **Zygotene**: Homologous chromosomes begin to pair up.
- **Pachytene**: This is where bivalent formation occurs.
- **Diplotene**: Homologous chromosomes start to separate but remain attached at chiasmata.
- **Diakinesis**: Chromosomes condense further and prepare for metaphase.

4. **Identifying Bivalent Formation**: Bivalent formation, or chromosome synapsis, specifically occurs during the **Pachytene** stage of Prophase I. During this stage, homologous chromosomes align side by side, forming pairs known as bivalents.

5. **Significance of Bivalent Formation**: This process is crucial as it facilitates crossing over, which is the exchange of genetic material between non-sister chromatids, leading to genetic diversity.

6. **Conclusion**: Therefore, the correct answer to the question is that chromosome synapsis or bivalent formation occurs in the **Pachytene** phase of Prophase I during meiosis.

### Final Answer:
Chromosome synapsis or bivalent formation occurs in the **Pachytene** phase of Prophase I during meiosis.

---