Question

Number of bivalents, chromatid tetrads, telomeres present in pachytene phase of meiocyte of maize is
A. $20,20,80,80$
B. $20,20,40,80$
C. $10,10,40,80$
D. $10,20,40,40$

Answer 1

Hint:- Meiosis I start with the prophase of the first meiotic division is typically longer and more complex when compared to the prophase of mitosis. It had been further subdivided into five stages based on chromosomal behavior leptotene, zygotene, pachytene, diplotene, & diakinesis.

Complete step-by-step solution:- Maize has a chromosome number of $20\left( {2n} \right)$. In the pachytene stage, a tetrad of the chromosome will form known as bivalent. This is the stage when the homologous recombination and chromosomal crossover takes place. Here the DNA molecules are packed into thread-like structures called chromosomes. A bivalent chromosome or sister chromatids is one pair of chromosomes in a tetrad. A tetrad is an association of a pair of homologous chromosomes that are held together physically during DNA crossover. This physical attachment allows for the alignment and segregation of the homologous chromosomes in the first meiotic division. Telomeres are caps at the end of each strand of DNA that protect our chromosomes like plastic tips at the end of shoelaces.
So, in maize, the number of bivalents and tetrads is $20$. Each chromatid has one DNA strand. So, in $40$ chromatids in DNA $20 \times 2 = 40$, Each DNA has $2$ telomeres. So, telomeres are $40 \times 2 = 80$. So, the correct answer is option B ($20,20,40,80$)

Note:-
During the pachytene stage bivalent chromosomes now clearly appear as tetrads. This stage is characterized by the appearance of recommendation modules at which crossing over takes place between non-sister chromatids of homologous chromosomes. Crossing over is nothing but an exchange of genetic material.