Definition
Sister chromatids are the two DNA copies produced when a single chromosome is replicated during S phase of the cell cycle. They remain physically attached (primarily by cohesin proteins) and are joined most tightly at the centromere until the appropriate stage of cell division.
Key clarification: chromosome vs chromatid
- Before DNA replication: one chromosome consists of one DNA double helix (one chromatid).
- After DNA replication (S phase complete): one chromosome consists of two sister chromatids attached at the centromere.
- After sister chromatids separate: each separated chromatid is considered an individual chromosome again.
Visualization: a replicated chromosome and its sister chromatids
How sister chromatids are produced
Step 1: DNA replication in S phase
During S phase, each chromosome’s DNA is copied by semi-conservative replication. This creates two DNA molecules with the same genetic sequence (except for rare replication errors), packaged as two sister chromatids.
Step 2: Cohesion holds the copies together
Protein complexes (especially cohesin) link the sister chromatids along their length and at the centromere. This physical coupling is essential for correct alignment and segregation on the mitotic or meiotic spindle.
What happens to sister chromatids during mitosis and meiosis
| Process / stage | What aligns at the metaphase plate | What separates at anaphase | Resulting genetic outcome |
|---|---|---|---|
| Mitosis (metaphase → anaphase) | Individual replicated chromosomes (each with two sister chromatids) | Sister chromatids | Two genetically similar daughter cells (variation mainly from mutation) |
| Meiosis I (metaphase I → anaphase I) | Homologous chromosome pairs (tetrads) | Homologous chromosomes (sister chromatids stay together) | Reduction to haploid sets; independent assortment contributes variation |
| Meiosis II (metaphase II → anaphase II) | Replicated chromosomes in each haploid cell | Sister chromatids | Four haploid products; chromatids become separate chromosomes |
Critical genetics connection: crossing over involves non-sister chromatids
In prophase I of meiosis, homologous chromosomes pair and exchange DNA segments by crossing over. The exchange occurs between non-sister chromatids (one chromatid from each homolog), not between sister chromatids, which are already copies of the same chromosome. This is one reason sister chromatids are central to linkage and recombination discussions.
Counting chromosomes and chromatids (common exam-style application)
A frequent misconception is that “chromosome number doubles after S phase.” The chromosome count is defined by the number of centromeres, not the amount of DNA.
Example
A diploid cell has \(2n = 6\) chromosomes in G1. Determine the number of chromosomes and chromatids after S phase (G2).
Step-by-step solution
- Chromosomes are counted by centromeres. Replication does not create new centromeres.
- Therefore, after S phase the number of chromosomes remains \(6\).
- Each chromosome now has two sister chromatids, so the number of chromatids is: \[ \text{chromatids} = 2 \cdot \text{chromosomes} = 2 \cdot 6 = 12 \]
After S phase: chromosomes stay the same in number, DNA content doubles, and sister chromatids appear (two chromatids per chromosome).
Summary
Sister chromatids are the paired, replicated copies of a chromosome formed in S phase and held together at the centromere by cohesion. They separate in anaphase of mitosis and in anaphase II of meiosis, and they provide the structural context needed to distinguish sister vs non-sister chromatids in recombination and linkage concepts.