Final answer:
The two chromatids of a chromosome are joined together at the centromere. This region is where cohesin proteins hold the sister chromatids together until they are separated during cell division.
Step-by-step explanation:
The two chromatids of a chromosome are joined together at a region of DNA called the centromere. DNA replication occurs in the S phase of interphase and not during mitosis.
Post replication, each chromosome consists of two sister chromatids that are identical. These chromatids are held together at the centromere by cohesin proteins. The centromere is a highly condensed region, making it visible as a constricted area when viewed under a light microscope.