At the beginning of the S phase, the cell’s nucleus has two pairs of each chromosome, one pair member from each parent. Histone proteins surround the long, linear double helix structures at the heart of the chromosome. Replication creates a second linear structure, or chromatid, attached to the original one all along its length by the protein cohesion. The resulting double-chromatid chromosomes will assume the familiar X-shaped appearance when they condense at the start of mitosis, held together by only a globular centromere. Later in mitosis, the centromeres dissolve, freeing the chromatids that then become independent chromosomes that are distributed to each daughter cell.
