Name the event during cell divison cycle that results in the gain or loss of chromosome.

Explanation

### Step-by-Step Solution:

1. **Understanding Cell Division**: The cell division cycle consists of several phases, including interphase (G1, S, G2) and mitosis (M phase). During these phases, the chromosomes are replicated and distributed to daughter cells.

2. **Identifying the Event**: The specific event during the cell division cycle that leads to the gain or loss of chromosomes is known as **aneuploidy**. Aneuploidy occurs when there is an abnormal number of chromosomes in a cell.

3. **Mechanism of Aneuploidy**: Aneuploidy can happen due to errors in the segregation of chromosomes during mitosis or meiosis. This can result in either:
- **Gain of chromosomes**: This is when a cell has more than the normal number of chromosomes (e.g., trisomy).
...