Final answer:
The S phase in cell division is the stage in which DNA synthesis occurs, resulting in the doubling of the nuclear DNA content and sister chromatids formation.
Step-by-step explanation:
In cell division, the S phase consists of DNA synthesis and the nuclear DNA content doubles. During the S phase of the cell cycle, each chromosome's DNA is replicated to form two identical copies known as sister chromatids, which are connected at a region called the centromere. This replication ensures that when the cell divides, both daughter cells receive an exact copy of the DNA. The S phase is part of a larger process called interphase, which also includes the G1 and G2 phases, where the cell grows and prepares for division. The centrosome is also duplicated during the S phase, giving rise to the mitotic spindle, essential for chromosome separation during mitosis.