Question

Find out the number of chromosomes and the amount of DNA in a spore mother cell of bryophytes in $G2$ stage, if the spore contains $5$ chromosomes and $5$ $picograms$ of DNA.
A. $5$ chromosomes, $5$$pg$ DNA
B. $10$ chromosomes, $20$$pg$ DNA
C. $10$ chromosomes, $10$$pg$ DNA
D. $20$ chromosomes, $20$$pg$ DNA

Answer 1

Hint: There are two stages in the life cycle of bryophytes. The plant body is called a gametophyte that produces haploid gametes which fuse together to form a zygote. A zygote is diploid and it produces sporophyte. The sporophyte produces haploid spores and these spores germinate and form the gametophyte.

Complete answer:
The spores are produced from diploid sporophyte after reduction division, these spores are haploid in nature. The reduction division or meiosis is an important step in sexual reproduction in the majority of organisms. Meiosis is a special type of cell division in which chromosome number is halved to produce haploid daughter cells from diploid cells. During the interphase, the amount of DNA is doubled while the number of chromosomes is unchanged. It then undergoes meiosis-I. After this stage, the amount of DNA will be half of that in the interphase. The number of chromosomes is halved along with this. In other words, if the amount of DNA was $2C$ before entering the cell division, it becomes $4C$ in interphase and again becomes $2C$ after meiosis-I. Meanwhile, if the number of chromosomes was $2n$ before entering cell division, it is unchanged in interphase but is halved after meiosis-I and becomes $n$. After meiosis $2$, the amount of DNA is again reduced by half and becomes $1C$, while the chromosome number is the same, that is $1n$. After meiosis-II, we can see that both chromosome number and amount of DNA is reduced by half from that of the mother cell. While comparing the amount of DNA in the $G2$ phase, which is in the interphase, DNA content is decreased by four times, and the number of chromosomes is just halved. As the spore contains $5$ chromosomes and $5$ $picograms$ of DNA, the spore mother cell in the $G2$ phase would have $10$ chromosomes and $20$ $ picograms$ of DNA.
Hence, the correct option is B.

Note: Meiosis division happens in two cycles. They are meiosis-I and meiosis-II. Although meiosis is different from mitosis, the second step of meiosis is similar to mitosis. Mitosis is also called an equational cell division.