doublechromatid

Double chromatid refers to a replicated chromosome that consists of two sister chromatids held together at the centromere. This state arises during the S phase of the cell cycle when the DNA is duplicated. Each chromatid contains an identical DNA molecule; the two are genetically identical barring rare replication errors. The sister chromatids are bound by cohesin protein complexes and are held in close association until they are segregated during mitosis or meiosis.

During mitosis, chromosomal condensation occurs and kinetochores assemble at the centromeres to facilitate attachment to spindle fibers. In metaphase, the double chromatids align at the metaphase plate; during anaphase, separase cleaves cohesin, allowing sister chromatids to separate and move to opposite poles as daughter chromosomes.

In meiosis, the concept is similar: after replication, each chromosome consists of two sister chromatids; homologous chromosomes pair and recombine during prophase I, but the centromeric cohesion is maintained to separate sister chromatids during meiosis II. Observations of replicated chromosomes by microscopy often depict X-shaped chromosomes; unreplicated chromosomes appear as single structures (dyads). Proper regulation of cohesion and its release is critical for accurate chromosome segregation; defects can cause aneuploidy or genomic instability.