Sister Chromatids

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By EUGENE M. MCCARTHY, PHD GENETICS Search the dictionary >>

Chromosome pairs
Chromosomes are paired, and each replicated chromosome is itself composed of a pair of chromatids. (Enlarged image)
Diagram of Chromosome Structure
Chromosome structure
(Enlarged image)
Note: After separation, the two former chromatids are called "unreplicated chromosomes."
sister chromatids

A single replicated chromosome has two chromatids. One chromatid is shown in yellow, the other in blue.
Sister chromatids (pronounced "KRŌ-mə-tidz") are the two identical halves of a single replicated eukaryotic chromosome. They are joined at the centromere (see diagram at right) and genetically identical because, during interphase, they are produced from the two complementary strands of a single duplex DNA molecule (more about replication >>).

So any given locus on one chromatid always has the same allele as the one present at the same locus on that chromatid's sister (except after crossing-over during meiosis). In contrast, two homologous chromosomes usually do not have identical alleles at all loci. They are inherited from different parents and therefore are not derived from the same DNA molecule.

The two sister chromatids of each chromatid pair are segregated into separate cells in both mitosis and meiosis. But they remain together throughout the first division during meiosis (meiosis I). It is only during the second meiotic division (during anaphase II) that they finally separate.
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