Figure 1.

Chromosome organization during mitosis. During the cell cycle, a chromosome is replicated during S phase. The duplicated copies of the chromosome are called sister chromatids (black and grey). Sister chromatids are held together from the time of replication in S phase until the onset of chromosome segregation during mitosis. This cohesion facilitates bipolar attachment of the sister kinetochores to the microtubules of the mitotic spindle, ensuring the orderly segregation of sister chromatids to opposite poles. At the onset of mitosis, sister chromatids become visibly condensed. Chromosome condensation separates sister chromatids into distinct domains, which facilitates subsequent sister chromatid segregation by the mitotic spindle. Condensation also shortens the length of the sister chromatids, which ensures that they are properly packaged into the dividing cell upon cytokinesis.