Chromatid

A chromatid is one of the two identical copies of a chromosome that result from DNA replication during the S phase of the cell cycle. The two copies are termed sister chromatids and are held together at the centromere, forming a duplicated chromosome. Each chromatid contains a complete copy of the chromosome’s DNA sequence and associated proteins.

The connection between sister chromatids is maintained by cohesin protein complexes. The centromere region typically provides a site for the assembly of the kinetochore, a protein structure that attaches to microtubules of the spindle apparatus. This attachment enables the spindle to pull the sister chromatids apart during cell division.

In mitosis, sister chromatids separate during anaphase, and each chromatid is pulled toward opposite poles, ultimately becoming an independent chromosome in the daughter cells. In meiosis, the behavior is slightly different: homologous chromosomes are first separated during meiosis I, while sister chromatids remain held together through meiosis I and separate during meiosis II.

Chromatid versus chromosome: a chromosome can refer to a single chromatid before replication or to a duplicated chromosome consisting of two sister chromatids after replication. Once the centromeres split, each chromatid is considered an individual chromosome. Chromatids thus play a central role in accurate genetic distribution during cell division.