Cromátidas

Cromátidas are one of two identical halves of a replicated chromosome. During cell division, specifically in the S phase of the cell cycle, a chromosome undergoes DNA replication. This process results in the formation of two identical copies of the original chromosome, each attached at a central region called the centromere. These two identical copies are the sister chromatids.

Before replication, a chromosome exists as a single DNA molecule. After replication, it consists of two identical DNA molecules, each forming a chromatid. These sister chromatids are held together by proteins, primarily cohesins, at the centromere. Once cell division begins, specifically during anaphase of mitosis and meiosis II, the sister chromatids are separated and are then considered individual chromosomes.

If a cell has 2n chromosomes, after replication it will have 2n chromosomes, each composed of two sister chromatids. When these sister chromatids separate, the cell will briefly contain 4n individual chromosomes before it divides into two daughter cells, each with 2n chromosomes. The term chromatid is also used for non-replicated chromosomes, where it refers to a single DNA molecule and its associated proteins. However, the more common usage refers to the identical halves of a replicated chromosome.