Definition of Chromatid
A chromatid (Greek khrmat- '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 copieseach forming one half of the replicated chromosomeare called chromatids. During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes.
Chromatid pairs are normally genetically identical, and said to be homozygous; however, if mutation(s) occur, they will present slight differences, in which case they are heterozygous. The pairing of chromatids should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome. Chromonema is the fibre-like structure in prophase in the primary stage of DNA condensation. In metaphase, they are called chromatids.