Final answer:
Cohesin forms a physical ring around sister chromatids at the centromere, holding them together until they are separated during cell division.
Step-by-step explanation:
Cohesin forms a physical ring around sister chromatids at a region known as the centromere. During the S phase of interphase, DNA replicates to form chromatids, which are identical strands of DNA. These chromatids are held together by cohesin, making them sister chromatids. The cohesin proteins ensure that when the chromosomes condense during mitosis, the identically packed sister chromatids remain connected until they are separated into individual chromosomes during cell division.