AP BIOLOGY

Detailed explanation-1: -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.

Detailed explanation-2: -The S phase of a cell cycle occurs during interphase, before mitosis or meiosis, and is responsible for the synthesis or replication of DNA. In this way, the genetic material of a cell is doubled before it enters mitosis or meiosis, allowing there to be enough DNA to be split into daughter cells.

Detailed explanation-3: -Then, at a critical point during interphase (called the S phase), the cell duplicates its chromosomes and ensures its systems are ready for cell division. If all conditions are ideal, the cell is now ready to move into the first phase of mitosis.

Detailed explanation-4: -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).

Detailed explanation-5: -In the S phase (synthesis phase), DNA replication results in the formation of two identical copies of each chromosome-sister chromatids-that are firmly attached at the centromere region. At this stage, each chromosome is made of two sister chromatids and is a duplicated chromosome.