Which of the following events takes place during Pachytene of Prophase I?

Options

A. Dissolution of synaptonemal complex
B. Formation of synaptonemal complex
C. Condensation of chromosomes and crossing over
D. Disappearance of nucleolus and nuclear membrane

Answer

To answer the question about which event takes place during Pachytene of Prophase I, we will follow these steps:

### Step 1: Understand the stages of Prophase I
Prophase I of meiosis consists of several stages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis. Each of these stages has distinct events that occur.

### Step 2: Identify the events in each stage
- **Leptotene**: Chromosomes begin to condense and become visible.
- **Zygotene**: Homologous chromosomes pair up and form the synaptonemal complex.
- **Pachytene**: This is where crossing over occurs between non-sister chromatids.
- **Diplotene**: The synaptonemal complex begins to dissolve, and chiasmata become visible.
- **Diakinesis**: Chromosomes condense further, and the nuclear envelope breaks down.

### Step 3: Focus on Pachytene
During the Pachytene stage, the key event is the crossing over of genetic material between non-sister chromatids. This process increases genetic diversity.

### Step 4: Analyze the options provided
1. **Dissolution of Synaptonemal Complex**: This occurs in Diplotene, not Pachytene.
2. **Formation of Synaptonemal Complex**: This occurs in Zygotene, not Pachytene.
3. **Condensation of Chromatin and Crossing Over**: This is the correct event that occurs during Pachytene.
4. **Disappearance of Nucleolus and Nuclear Membrane**: This occurs in Leptotene.

### Conclusion
The correct answer is that during Pachytene of Prophase I, the event that takes place is the **condensation of chromatin and crossing over** between non-sister chromatids.