Chromatid 染色单体
A chromatid (Greek khrōmat- 'color' + -id) is one copy of a newly copied chromosome which is still joined to the other copy by a single centromere.
Before replication, one chromosome is composed of one DNA molecule. Following replication, each chromosome is composed of two DNA molecules; in other words, DNA replication itself increases the amount of DNA but does not increase the number of chromosomes. The two identical copies—each forming one half of the replicated chromosome—are called chromatids. During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes.