When do homologous chromosomes pair up

Options

A. Only in mitosis
B. Only in meiosis I
C. Only in meiosis II
D. In both mitosis and meiosis.

Answer

To answer the question "When do homologous chromosomes pair up?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Chromosome Types**:
- Homologous chromosomes are pairs of chromosomes, one from each parent, that are similar in shape, size, and genetic content.

2. **Identifying the Process**:
- Chromosome pairing occurs during cell division. There are two main types of cell division: mitosis and meiosis.

3. **Focusing on Meiosis**:
- Meiosis is the process that leads to the formation of gametes (sperm and egg cells) and consists of two rounds of division: Meiosis I and Meiosis II.

4. **Meiosis I - Prophase I**:
- During Prophase I of meiosis, homologous chromosomes undergo synapsis, where they pair up closely to form structures known as bivalents or tetrads.

5. **Metaphase I**:
- In Metaphase I of meiosis, these homologous chromosome pairs (bivalents) align at the metaphase plate. This is the key stage where the pairing of homologous chromosomes is evident.

6. **Genetic Variation**:
- The random orientation of these pairs during Metaphase I contributes to genetic variation in the resulting gametes.

7. **Conclusion**:
- Therefore, homologous chromosomes pair up only during Meiosis I, specifically during Prophase I and Metaphase I.

### Final Answer:
Homologous chromosomes pair up only in Meiosis I.

---