A chromatid can be thought of as one half of a replicated chromosome. When DNA is replicated, during the interphase of the cell cycle, a single chromosome is duplicated and each of the duplicates are called sister chromatids. The sister chromatids are attached at a centromere. During anaphase of mitosis, and anaphase II of meiosis, sister chromatids are separated by attachment of their individual kinetochore proteins to kinetochore microtubules (originating from centrosomes), which then depolymerise to pull them apart. Once pulled apart, they are known as sister chromosomes and will replicate themselves again during interphase in the next generation of cells.

A dyad is the structure found in Prophase I of meiosis: two sister chromatids attached by a seemingly undivided centromere. This is because the aim of meiosis I is only to separate pairs of homologous chromosomes, so the sister chromatids on an individual chromosome need to stay intact with one another. When two dyads come together, in a homologous pair, the structure is called a tetrad. In mitotic prophase (as well as prophase II of meiosis), by contrast, the sister chromatids have their own centromeres with independent kinetochores, allowing the chromatids to be separated from one another.

Sister chromatids should not be confused with homologous chromosomes. Sister chromatids are identical DNA sequences formed by DNA replication, whereas homologous chromosomes are chromosomes that carry the same genes as each other, though not always the same alleles as each other (i.e. they often have different DNA sequences). Homologous chromosomes originate from different parents, not from DNA replication. The ploidy of a cell refers to the number of homologous versions of each chromosome there are - it has nothing to do with sister chromatids. So a cell with two homologous versions of each chromosome is diploid; it does not become tetraploid when each of those chromosomes replicates to form sister chromatids. While homologous chromosomes are separated in meiosis I, sister chromatids are separated in mitosis or meiosis II.