Chromatid: structure, function and role in cell division

Structure and components

Each chromatid contains an identical sequence of DNA packaged with histone and non-histone proteins to form chromatin. The DNA molecule in a chromatid carries the same genetic information as its sister; chromatin organization, higher-order folding and associated proteins determine how compact or accessible that DNA is in different cell-cycle stages. Core components include the DNA double helix (DNA), chromatin fibers (chromatin), and protein complexes that regulate cohesion and separation (proteins).

Behavior in the cell cycle and mitosis

Chromatids form during the S phase when each chromosome is replicated. After replication the two sister chromatids remain paired until cell division. During mitosis they condense and align at the metaphase plate; when the centromere divides, the sister chromatids are pulled apart to opposite poles and are then considered individual chromosomes. Key stages in chromatid behavior include:

• Replication in S phase.
• Cohesion maintenance until anaphase.
• Separation when centromeres split, producing two daughter chromosomes.

Meiosis, crossing over and genetic recombination

Meiosis differs from mitosis in that homologous chromosomes pair and exchange segments during prophase I. Crossing over occurs between non‑sister chromatids of homologous chromosomes; these physical exchanges create new combinations of alleles and increase genetic diversity in gametes. After the first meiotic division sister chromatids may still remain together, but ultimately recombined chromatids segregate into haploid cells, contributing to variation among sperm or eggs (meiosis) and thus to the diversity of gametes (gametes).

Distinctions and biological importance

It is important to distinguish sister chromatids (copies of the same chromosome made by replication) from non‑sister chromatids (from homologous chromosomes). Before centromere separation they are counted as one replicated chromosome; after separation each chromatid is termed a chromosome in its own right. Errors in chromatid separation, such as nondisjunction, can produce aneuploid gametes and have significant biological consequences. Chromatids are routinely observed in cytogenetics and cell biology to study segregation, recombination and chromosomal abnormalities.