Question

In meiosis I, a bivalent is an association of
A. Four chromatids and four centromeres
B. Two chromatids and two centromeres
C. Two chromatids and one centromeres
D. Four chromatids and two centromeres

Answer 1

Hint:-Pairing of homologous chromosomes is a defining characteristic of meiosis and distinguishes meiosis from mitosis. Bivalent is a pair of homologous chromosomes lying together in the zygotene stage of prophase I of first meiotic division.

Complete Answer:-A bivalent is the association of two replicated homologous chromosomes having exchanged DNA strand in at least one site called chiasmata. Each bivalent contains a minimum of one chiasma and rarely more than three. This limited number is due to crossover interference, a poorly understood phenomenon that limits the number of resolution of pair events into crossover in the vicinity of another pre –existing crossover outcome, thereby limiting the total number of crossover per homologous pair. The formation of bivalent occurs during the first division of meiosis in the stage of prophase I. At the meiotic metaphase I, the cytoskeleton puts the bivalents under tension by pulling each homology in the opposite direction.
The term bivalent is used to denote a pair of homologous chromosomes. Each chromosome of such a pair is made up of two chromatids joined together at the centromere. Thus, each bivalent is actually made up of four chromatids and two centromeres.

Note:- A bivalent is one pair of chromosomes ( sister chromatids ) in a tetrad. A tetrad is the association of a pair of homologous chromosomes ( four sister chromatids ) physically held together by at least one DNA chromatid. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division.