chromatidy

Chromatids are one of two identical halves of a replicated chromosome. During cell division, specifically in the S phase of the cell cycle, a chromosome duplicates itself. Each of these identical copies is called a chromatid. Before replication, a chromosome exists as a single strand of DNA. After replication, the chromosome consists of two sister chromatids, which are joined together at a region called the centromere. These sister chromatids are exact copies of each other, containing the same genetic information.

The term "chromatid" is primarily used when discussing chromosomes during mitosis and meiosis, the processes of cell division. When a cell prepares to divide, the replicated chromosomes condense and become visible under a microscope. At this stage, each chromosome appears as an X-shape, with the two sister chromatids forming the arms of the X. During anaphase of mitosis and anaphase II of meiosis, the sister chromatids separate and are then considered individual chromosomes. Each separated chromatid, now a full chromosome, moves to opposite poles of the cell. The term "sister chromatids" is used to emphasize their identical nature and shared origin from a single replicated chromosome. Non-sister chromatids, found on homologous chromosomes, are not identical and are involved in processes like crossing over during meiosis.