Final answer:
Each chromosome is comprised of two identical sister chromatids joined at the centromere before cell division. These chromatids are the result of DNA replication during the S phase and are separated into daughter cells during mitosis or meiosis.
Step-by-step explanation:
Prior to nuclear division such as mitosis or meiosis, each chromosome is composed of two genetically identical parts called sister chromatids. These sister chromatids are connected by a specific region known as the centromere. During the S phase of the cell cycle, DNA replication takes place, resulting in each chromosome being copied to produce two chromatids that remain attached at the centromere. The cohesin proteins are crucial as they hold the chromatids together, ensuring their proper separation during the later stages of cell division. Ultimately, they're essential in generating genetically identical or diverse daughter cells, depending on whether the process is mitosis or meiosis.
During mitosis, sister chromatids are separated and distributed into two daughter cells, retaining the original chromosome number. In contrast, during meiosis, homologous chromosomes initially segregate, then sister chromatids separate, leading to haploid cells that contribute to genetic diversity through sexual reproduction.