Question

The nucleus of a diploid organism contains $3$ng of DNA in $G_1$ phase. Which one of the following statements describes the state of the cell at the end of S phase?

• A. The nucleus divides into two, and each nucleus contains $3$ng of DNA.
• B. The nucleus does not divide, and it contains $3$ng of DNA.
• C. The nucleus divides into two, and each nucleus contains $1.5$ng of DNA.
• D. The nucleus does not divide, and it contains $6$ng of DNA.

Answer 1

The correct option is D The nucleus does not divide, and it contains $6$ng of DNA.

During G1 phase, the cell grows and makes a variety of proteins that are needed for DNA replication. During S phase, which follows G1 phase, all the chromosomes are replicated. Following replication, each chromosome now consists of two sister chromatids. Thus, the amount of DNA in the cell has effectively doubled, even though the ploidy, or chromosome count, of the cell remains at 2n. Chromosomes double their number of chromatids post replication but the nuclei remains diploid as the number of centromeres and chromosomes remains unchanged. Hence, there is no division of the nucleus but there is a change in the amount of DNA in the cell (it is doubled), i.e. it changes to 6ng from 3ng.

So, the answer is D.