sister chromatids

When a chromosome is copied in preparation for cell division, the original and its fresh duplicate stay joined together for a while, like two identical photocopies still stapled at one corner. These two attached copies are called sister chromatids.

The two sisters are produced by DNA replication, so they carry identical DNA sequences (aside from any rare copying error). They remain held together along their length and especially at the centromere by ring-shaped protein complexes, giving a duplicated chromosome its familiar X shape. They count as a single chromosome while joined, not as two.

Sister chromatids exist only between the time the DNA is copied and the time the cell pulls them apart. In ordinary cell division (mitosis) the two sisters are separated and one goes to each daughter cell, so the cells stay genetically identical. In meiosis the sisters stay together through the first division and separate only in the second, a delay that is key to producing gametes with half the usual number of chromosomes.