singlechromatid

A single chromatid refers to a chromosome that consists of a single DNA molecule. This unreplicated form is present during the early stages of the cell cycle, most notably in the G1 phase, before DNA replication has occurred.

After the S phase of the cell cycle, each chromosome is replicated to form two sister chromatids that are held together at the centromere. The resulting structure is a double-chromatid chromosome, with two identical DNA molecules. Cohesin proteins help keep the sister chromatids attached until they are separated during cell division.

In mitosis, double-chromatid chromosomes condense and align at the metaphase plate, then separate at anaphase as the sister chromatids are pulled toward opposite poles. After separation, each chromatid behaves as an individual chromosome in the daughter cells. In meiosis, chromosomes are replicated to form sister chromatids; during meiosis I, homologous chromosomes are separated while sister chromatids remain attached, and during meiosis II, the centromeres split and the chromatids are finally separated into independent chromosomes.

The term single chromatid is commonly used to describe chromosomes before DNA replication, to emphasize the unreplicated state. Understanding the distinction between single-chromatid and double-chromatid chromosomes helps explain the mechanics of chromosome segregation and genetic inheritance during cell division.

See also: Chromosome; Chromatid; Sister chromatid; Centromere; Cohesin; Cell cycle.