DNA molecules replicate to form identical chromatids

Each chromosome divides longitudinally into two chromatids (left); at the molecular level (right), the single DNA molecule of each chromosome replicates, producing two DNA molecules, one for each chromatid. Also shown are various combinations of a gene with wild-type allele b+ and mutant form b, caused by the change in a single base pair from GC to AT. Notice that, at the DNA level, the two chromatids produced when a chromosome replicates are always identical with each other and with the original chromosome.