chromatid, one of a pair of daughter strands of a replicated chromosome. Chromatids serve an essential role in cell division, ensuring the accurate division and distribution of chromosomes to new daughter cells.
Chromatids are formed during chromosome duplication, which occurs prior to cell division via the processes of mitosis and meiosis. The two “sister” chromatids in a pair are identical and are joined by a centromere. The centromere is the point of attachment of the kinetochore, a protein structure that is connected to the spindle fibres. The spindle fibres pull the chromatids to opposite ends of the cell, causing each chromatid pair to separate; each chromatid becomes a separate chromosome at this point. In mitosis, the cell then divides, forming two daughter cells with a complete (diploid) set of chromosomes.
In meiosis, following the first round of cell division, homologous recombination occurs, in which genes are exchanged between a maternal chromatid and a paternal chromatid of a homologous chromosome pair. Following a second round of cell division, four haploid daughter cells are produced; thus, each new cell contains only one chromatid, or half of the complete chromosome complement.
Chromatids can be affected by various aberrations, resulting in significant genetic and cellular defects. For example, failure of chromatids to separate properly during cell division can result in an incorrect chromosome number in affected daughter cells, leading to aneuploidy (atypical chromosome number). Defects in sister chromatid cohesion are associated with the development of certain types of cancer.