Which of the events listed below is not observed during mitosis?

Options

A. Chromatin condensation
B. Movement of centrioles to opposite poles
C. Appearance of chromosomes with two chromatids joined together at the centromere
D. Crossing over

Answer

To determine which event is not observed during mitosis, let's analyze the options provided:

1. **Chromatin condensation**: This is a key event that occurs during mitosis. Chromatin condenses to form visible chromosomes, making it easier for the cell to segregate genetic material.

2. **Movement of centrioles to opposite poles**: This is also observed during mitosis. Centrioles move to opposite ends of the cell to help organize the mitotic spindle, which is crucial for the separation of chromosomes.

3. **Appearance of chromosomes with two chromatids joined together at the centromere**: This event occurs during mitosis as well. After DNA replication, chromosomes consist of two sister chromatids connected at the centromere, which are separated during the later stages of mitosis.

4. **Crossing over**: This event is not observed during mitosis. Crossing over occurs during meiosis, specifically in prophase I, where homologous chromosomes exchange genetic material. Mitosis does not involve this process as it is focused on producing two identical daughter cells.

Based on this analysis, the event that is **not observed during mitosis** is **crossing over**.

### Step-by-Step Solution:
1. **Identify the events listed**: Look at the four options provided.
2. **Analyze each event**:
- Chromatin condensation: Confirmed to occur during mitosis.
- Movement of centrioles: Confirmed to occur during mitosis.
- Appearance of chromosomes with two chromatids: Confirmed to occur during mitosis.
- Crossing over: Confirmed to occur during meiosis, not mitosis.
3. **Determine the answer**: The event that does not occur during mitosis is crossing over.