Question

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

Answer 1

Hint: A bivalent is one pair of tetrads that is associated with a pair of homologous chromosomes. The formation of a bivalent occurs during its first division. Bivalent is the association of two replicated homologous chromosomes.

Complete answer:
Option A (i), (ii), (iii), and (iv):
Bivalent occurs in the first division of meiosis. A bivalent is the association of two replicated homologous chromosomes having exchanged DNA strands. Bivalent is one pair in a tetrad. Bivalent is an association of two replicated chromosomes.
Option A is the correct answer.
Option B (iii) only - One bivalent consists of $$2$$ homologous chromosomes: Even though this fact is true about bivalent. But the other options are also true about bivalent. One bivalent consists of $$2$$ homologous chromosomes. The structure visible by microscopy is called bivalent. Bivalent are tetrads and they have two chromatids and $$2$$ centromeres. One bivalent consists of $$2$$ homologous chromosomes and these bivalents form in zygotene. These are also the features of Bivalent.
Option B is the wrong answer.
Option C (iii) and (iv): Even though both (iii) and (iv) are true in respect of bivalent. The other points are also true relating to bivalent. One bivalent consists of $$2$$ homologous chromosomes. Thus, it is true that one bivalent contains two chromosomes that are visible under microscopy. Bivalents form in zygotene. This is also a true fact that $$30$$ bivalents are formed in the zygotene stage. Bivalent is tetrads. Since other points are also true this option is wrong.
Option C is the wrong answer.
Option D (iv) only: Bivalents form in zygotene. This is a true fact. Each bivalent is formed by fusing four chromosomes. $$30$$ bivalents are formed during the zygotene stage. This option is wrong because the other points given are also related to bivalent. Bivalent are tetrads and they have two chromatids and $$2$$ centromeres. One bivalent consists of $$2$$ homologous chromosomes. Hence these are also correct regarding bivalent we can’t say only (iv) is correct regarding bivalent.
Option D is the wrong answer.

So, Option A is the correct answer.

Note:
Bivalents is one pair of chromosomes in a tetrad. Hence, we can say that bivalents are tetrads. A bivalent means $$4$$ chromatids and $$2$$ centromeres. One bivalent consists of $$2$$ homologous chromosomes.
Bivalents form in zygotene. $$30$$ bivalents are formed during zygotene.