Final answer:
In the S phase of interphase, a cell duplicates its chromosomes, making sister chromatids that are attached at the centromere. The centrosome, including centrioles, is also replicated to help in organizing cell division.
Step-by-step explanation:
During interphase, a cell duplicates its chromosomes and prepares for cell division. It is in the S phase (synthesis phase) of interphase that DNA replication occurs, resulting in two identical copies of each chromosome, known as sister chromatids. These sister chromatids are held together at a region known as the centromere. Concurrently, the centrosome is duplicated, which includes a pair of centrioles, crucial for organizing the mitotic spindle necessary for cell division during mitosis.