Which of these statements is not true?

Options

A. Crossing over is the exchange of genetic material between two homologous chromosomes.
B. The enzyme which is involved during crossing over is called recombinase.
C. Recombination between homologous chromosomes is completed by the end of diakinesis.
D. Synaptonemal complex is formed during zygotene.

Answer

To determine which statement is not true regarding meiosis and specifically prophase one, we can analyze each statement provided in the question:

1. **Crossing over is the exchange of genetic material between two homologous chromosomes.**
- This statement is true. Crossing over occurs during prophase one of meiosis, specifically in the pachytene stage, where homologous chromosomes exchange segments of genetic material.

2. **Enzyme involved during crossing over is called recombinance.**
- This statement is also true. The process of recombination during crossing over involves various enzymes, including those that facilitate the exchange of genetic material.

3. **Recombination between homologous chromosomes is completed by the end of diakinases.**
- This statement is **not true**. While recombination begins in the pachytene stage, it is largely completed by the end of the pachytene stage and into the diplotene stage, not diakinases. Diakinases is primarily associated with chromatin condensation and preparing for metaphase.

4. **Synaptonemal complex is formed during zygotene.**
- This statement is true. The synaptonemal complex forms during the zygotene phase of prophase one, facilitating the pairing of homologous chromosomes.

Based on the analysis, the statement that is not true is:

**"Recombination between homologous chromosomes is completed by the end of diakinases."**

### Step-by-Step Solution:
1. **Identify the statements related to meiosis.**
- Review each statement to understand its context within meiosis.

2. **Evaluate the truth of each statement.**
- Cross-check each statement with known facts about meiosis, particularly prophase one.

3. **Determine which statement is false.**
- Identify the statement that does not align with the established understanding of meiosis.

4. **Conclude with the incorrect statement.**
- Clearly state which statement is not true based on the evaluation.