Sister chromatids are the two identical DNA copies produced when a single chromosome replicates during S phase of the cell cycle. Each chromatid contains one continuous DNA double helix (packaged with histones), and the pair remains physically linked—most strongly at the centromere region—until separation during chromosome segregation.
Definition and structural features
A replicated chromosome consists of two sister chromatids held together by protein complexes (classically summarized as cohesin) that maintain alignment from S phase through early stages of division. The centromere is the chromosomal region where the kinetochore forms; microtubules attach there to move chromatids during segregation.
In standard cell-biology usage, a “chromosome” can refer to the unit defined by one centromere. A replicated chromosome still counts as one chromosome (one centromere) even though it contains two chromatids.
Sister chromatids versus homologous chromosomes
Sister chromatids are copies of the same chromosome produced by replication and are genetically identical along their length, aside from rare replication errors. Homologous chromosomes are a matched pair (one maternal, one paternal in diploids) that carry the same genes in the same order but can carry different alleles.
| Term | Relationship | Genetic similarity | Where the term matters most |
|---|---|---|---|
| Sister chromatids | Two copies of one chromosome after S phase | Essentially identical sequences | Mitosis; meiosis II; cohesion and segregation mechanics |
| Homologous chromosomes | Maternal–paternal pair of corresponding chromosomes | Same genes, possibly different alleles | Meiosis I; independent assortment; linkage framework |
| Non-sister chromatids | Chromatids belonging to different homologs | Similar gene map, alleles may differ | Crossing over and recombination in prophase I |
Replication, chromosome number, and DNA content
DNA replication doubles the amount of DNA without changing the chromosome count (when chromosomes are counted by centromeres). Using a diploid cell with \(2n\) chromosomes:
A concrete diploid example is \(2n=46\): after S phase, the cell still has 46 chromosomes (46 centromeres), but 92 chromatids and approximately doubled DNA content relative to G1.
Segregation timing in mitosis and meiosis
- Mitosis
- Sister chromatids separate at anaphase after bi-orientation, when cohesin at the centromere is removed and each chromatid becomes an independent daughter chromosome that moves to an opposite pole.
- Meiosis I
- Homologous chromosomes separate while sister chromatids remain together. This distinction underlies independent assortment and the logic of linkage: whole homologs partition, not sister chromatids.
- Meiosis II
- Sister chromatids separate in a mitosis-like division, producing haploid cells with unreplicated chromosomes.
Accurate visualization of sister chromatids in meiosis and mitosis
Relevance to linkage and independent assortment
Linkage and independent assortment are framed in terms of how alleles travel on homologous chromosomes through meiosis. Recombination that breaks linkage arises when crossing over exchanges DNA between non-sister chromatids of homologs, generating recombinant chromatids that remain as sister pairs until meiosis II. The timing matters: meiosis I partitions homologs (the context for independent assortment), while meiosis II partitions sister chromatids (the context for distributing the replicated chromatids into gametes).
Common pitfalls
X-shaped chromosomes are frequently counted as two chromosomes; under the centromere definition, an X-shaped structure is one replicated chromosome composed of two sister chromatids. Another frequent confusion is the word “identical”: sister chromatids begin as identical copies after S phase, but crossing over in meiosis I can make a chromatid a mosaic of maternal and paternal segments; the term “sister” still refers to the replication relationship, not to being unchanged by recombination.