AP BIOLOGY

Detailed explanation-1: -During S phase (synthesis phase), DNA undergoes replication and makes a copy of itself. Hence, the content of DNA doubles during S phase. Each chromosome after DNA replication will have two chromatids and there is no change in the chromosome number.

Detailed explanation-2: -During S phase, replication increases the DNA content of the cell from 2n to 4n, so cells in S have DNA contents ranging from 2n to 4n.

Detailed explanation-3: -During S phase, which follows G1 phase, all of the chromosomes are replicated. Following replication, each chromosome now consists of two sister chromatids (see figure below). Thus, the amount of DNA in the cell has effectively doubled, even though the ploidy, or chromosome count, of the cell remains at 2n.

Detailed explanation-4: -In S phase of the cell cycle the DNA content doubles, i.e., 1C to 2C for haploid cells and 2C to 4C (where “C” stands for “content"). Lately, however, this has not been a very commonly used representation of DNA content) for diploid cells. However, the number of chromosomes does not increase.

Detailed explanation-5: -During S phase, chromosomes replicate to give rise to two identical sister chromatids. These sister chromatids remain attached at their kinetochores. Hence there is no increase in number of chromosomes during S phase even though DNA replicates to increase its amount.