Question

How many numbers of DNA molecules form bivalent?
A. $2$
B. $4$
C. $6$
D. $3$

Answer 1

One pair of chromosomes or sister chromatids in a tetrad is termed as a bivalent. The association of a pair of homologous chromosomes which is actually $4$ sister chromatids, that are physically held together by at least one DNA crossover is called a tetrad. In the first meiotic division, this physical attachment allows for alignment and segregation of homologous chromosomes. During the first division of meiosis, the formation of bivalent occurs.

Complete answer:
Option A $2$: The bivalent is a pair of chromosomes or sister chromatids that are in a tetrad. The tetrad is an association of a pair of homologous chromosomes which means, $4$ sister chromatids. The total number of DNA molecules thus is $4$, not $2$.
Hence option A is not correct.
Option B $4$: The process of meiosis is characterized by the pairing of homologous chromosomes to form bivalents. A pair of homologous chromosomes are referred to as each bivalent. Each homologous pair consists of two sister chromatids that are joined at the centromere. Hence, each homologous pair consists of four chromatids, that are two pairs of sister chromatids joined at two distinct centromeres. In other words, each homologous pair comprises two pairs of non-sister chromatids. The total number of DNA molecules in a bivalent are four as a total of four chromatids are present in a bivalent.
Hence option B is the correct answer.
Option C $6$: During the stage of prophase of meiosis I, the homologous chromosomes pairs and forms synapses. These paired chromosomes are called bivalents. It has two chromosomes and four chromatids in which one chromosome is coming from each parent. Since the total number of DNA molecules formed are $4$.
So, option C is not correct.
Option D $3$: The bivalent is a pair of chromosomes in a tetrad. Tetrad is the association of a pair of homologous chromosomes which are $4$ sister chromatids that are physically held together by at least one of the DNA crossovers. There are $4$ DNA molecules in a bivalent.
Hence option D is not correct.

Therefore, Option B $4$ is the correct answer.

Note:
The resolution of DNA recombination intermediate into a crossover exchanges the DNA segments in-between two homologous chromosomes at a site known as a chiasma. This physical strand exchange and the cohesion between the sister chromatids along each chromosome are carried out to ensure the robust pairing of homologs in the diplotene phase. The structure is called a bivalent, which is visible by a microscope.