chromatids

Chromatids are the two identical copies of a chromosome that result from DNA replication. They remain joined at the centromere and are soon to be separated during cell division. Each replicated chromosome consists of two sister chromatids, which are genetically identical and together form a single replicated chromosome unit. The term chromatid can refer to either copy before separation; once sister chromatids separate, each is considered an individual chromosome.

Chromatids are held together by cohesin protein complexes that accumulate at the centromere. The centromere is the constricted region of the chromosome where kinetochores assemble and spindle fibers attach during mitosis and meiosis.

During mitosis, chromatids are aligned at the metaphase plate and then separated during anaphase as microtubules attached to kinetochores shorten. The separation distributes one chromatid to each daughter cell, so that each new cell inherits an identical set of chromosomes.

In meiosis, homologous chromosomes pair and may exchange genetic material through crossing over between non-sister chromatids in prophase I. This creates chromatids that are no longer all identical. Following the first division, the homologs separate, and later the sister chromatids finally separate during meiosis II, yielding four haploid gametes, each containing chromosomes consisting of single chromatids.