Question

A bivalent of Meiosis I consists of
A. Four chromatids and two centromeres
B. Two chromatids and one centromere
C. Two chromatids and two centromere
D. Four chromatids and four centromere

Answer 1

Hint: Meiosis I is a form of reductional division in which chromosomes in diploid cells segregate to form four haploid daughter cells. A Bivalent is a pair of homologous chromosomes that synapses in the Prophase I of Meiosis I.

Complete answer:
During Prophase I of Meiosis I, one chromosome from each parent side synapse between the leptotene and pachytene phase. This leads to the formation of a pair of homologous chromosomes that exchanges their DNA strand at one site which is called Chiasmata which is responsible for the alignment and segregation of the chromosomes. Bivalent is a pair of sister chromatids that forms a tetrad and lies together in the zygotene stage. Centromere links the pair of sister chromatids by attaching the spindle fibers at its specialized DNA sequence. They are located in the central area of the chromosome.
Since a bivalent has a pair of chromosomes, it will have 4 sister chromatids (2 in each chromosome) and 2 centromeres joining these 4 sister chromatids (1 in each chromosome). So, a Bivalent has 4 chromatids and 2 centromeres.
Meiosis I is also called a Reductional division because here the ploidy level is reduced to half while Meiosis II is called an equational division because the ploidy level remains the same.

Hence, the correct answer is option (A)

Note: The formation of a bivalent primarily occurs so that a physical attachment is formed which would further help in the alignment and the segregation of the chromosomes. This further aids the reduction process in this cellular division.