Which of the following is correct about bivalent?
(i) Bivalents are tetrads.
(ii) A bivalent means 4 chromatids and 2 centromeres.
(iii) One bivalent consists of 2 homologous chromosomes.
(iv) Bivalents form in zygotene.

Options

A. i,ii,iii and iv
B. iii only
C. iii and iv
D. iv only

Answer

To determine which statements about bivalents are correct, let's analyze each statement step by step:

1. **Statement (i): Bivalents are tetrads.**
- A bivalent is indeed a structure that consists of two homologous chromosomes that are paired together during meiosis. Each homologous chromosome is made up of two sister chromatids, resulting in a total of four chromatids. This structure is commonly referred to as a tetrad.
- **Conclusion:** This statement is correct.

2. **Statement (ii): A bivalent means 4 chromatids and 2 centromeres.**
- As established, a bivalent consists of two homologous chromosomes, each with one centromere. Therefore, a bivalent has a total of two centromeres and four chromatids (two from each homologous chromosome).
- **Conclusion:** This statement is correct.

3. **Statement (iii): One bivalent consists of 2 homologous chromosomes.**
- By definition, a bivalent is formed by the pairing of two homologous chromosomes. Each homologous chromosome contributes one chromatid from each of its two sister chromatids, resulting in the formation of a bivalent.
- **Conclusion:** This statement is correct.

4. **Statement (iv): Bivalents form in zygotene.**
- Bivalents form during the zygotene stage of prophase I of meiosis. It is at this stage that homologous chromosomes begin to pair up and form bivalents.
- **Conclusion:** This statement is correct.

**Final Conclusion:**
All four statements (i, ii, iii, and iv) about bivalents are correct.