Question

If there are 30 chromosomes in G1​ in phase, then what will be the number of bivalents in the zygotene stage?
A. 30
B. 15
C. 45
D. 60

Answer 1

Hint: The number of chromosomes remains constant (46 total chromosomes or 23 pairs of chromosomes) in all cells after every cell division. During interphase, the chromatids multiply to create 92 chromatids, but they're only found in pairs and are bound at the middle by the centromere.

Complete answer:
The chromatid pairs separate after mitosis, giving each daughter cell 46 chromatids.
Now comes the tricky part: meiosis. To ensure the right number of chromosomes, meiotic cells go through a more complicated phase.
Meiosis I: the homologous chromosomes divide in meiosis I. In this case, the chromatid pairs of the sister are still connected. At this stage, the cells of the daughter get a total of 23 chromosomes each and of 46 chromatids in total (which are actually 23 pairs of sister chromatids attached at the centromere). This is called reduction, since at this point the total number of chromosomes is halved.
Meiosis II: meiosis II is divided into just about a half as many chromosomes as mitosis. Thus the 23 chromatid pairs in the past cells (or a total of 46 chromatids) divided similarly, and so each daughter is exactly chromatid 23, which is 23 chromosomes respectively. Equational division is known as Meiosis II.
In spite of the fact that the 30 chromosomes from G stage will repeat during S-stage, they will be appended at the centromeric site by cohesin particles and consequently the quantity of chromosomes will stay 30. At the point when the zygotene stage starts, the homologous chromosomes pair up to frame bivalents. Subsequently, the 30 chromosomes will frame 15 bivalents/quadruplicates in the zygotene phase of prophase. Bivalent is shaped in the zygotene phase of prophase, where homologous chromosomes pair to frame the bivalent. Along these lines 15 bivalents will be framed, as the quantity of chromosome is 30.

Hence, the correct answer is option B-15.

Note:
To sum up, the total number of chromosomes in the cells of the daughter is unchanged in mitosis; while the total number of chromosomes in the daughter cells is halved in meiosis.