Question

A plant species has 10 pg DNA and 8 chromosomes in its pollen grain, then what will be the amount of DNA number of chromosomes in its microspore mother cell at $G_1$ phase.
A. 40pg; 16 chromosomes
B. 20pg; 16 chromosomes
C. 10pg; 8 chromosomes
D. 20pg; 32 chromosomes

Answer 1

Hint:- Pollen grains are the product of meiosis of the microspore mother cells. The microspore mother cell undergoes meiosis to form microspore tetrad which eventually develops into the pollen grain. Pollen grains are also called the male gametes.

Complete step-by-step solution:-
We know that in meiosis, the chromosome number present in the daughter cell is half that of the number of chromosomes present in the parent cell.
-Therefore in the case of a microspore mother cell which undergoes meiosis, the chromosome number gets halved and the pollen grains receive half the number of chromosomes.
-We also observe in cell division that the amount of DNA remains the same as the DNAreplicates ahead of the division to ensure an equal division of DNA, but in the case of meiosis, the amount of DNA is reduced to half as it goes through two cycles of division and DNA replication occurs only once.
-Therefore in the above question, we see that the number of chromosomes present in the pollen grains is 8 i.e. half the number of chromosomes present in the actual or the parent cell. Thus the number of chromosomes present in the microspore mother cells is 16(double).
-DNA replicates during the S phase of the cell cycle; as the $G_1$ phase lies ahead of the Sphase, hence the amount of DNA will be the same of the daughter cells but as there is a meiotic division, the amount of DNA present in the parent cell will double of that present in the daughter cell. Thus the amount of DNA will be 20pg.

So the correct option is B, ‘20pg; 16 chromosomes’

Note:-
One must be careful while calculating the amount of DNA in different types of cells. As in the case of mitosis, the amount of DNA transferred to the daughter cell from the mother cell is the same because DNA replication and cell division occur only once. Whereas in meiosis the division of cell is two times i.e. meiosis I and meiosis II resulting in four daughter cells, but the division of DNA occurs only once which prior to meiosis I. Hence in meiosis, only half the amount of total DNA present in the parent cell is passed on to the daughter cells.