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Sister Chromatids
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A single replicated chromosome has
two sister chromatids.
One chromatid is shown in yellow, the other in blue.
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A chromatid is one of the two halves of a replicated eukaryotic chromosome (see diagram at right). The two chromatids that make up a chromosome are called sister chromatids. They are joined at their centromeres and are genetically identical because, during interphase, they are each produced from one of the two strands of a single duplex DNA molecule (more about replication). Therefore any given locus on one chromatid always has the same allele as the one present at the same locus on that chromatid's sister. 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.
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Chromosomes are paired, moreover,
each replicated chromosome is itself
composed of a pair of chromatids.
(click to enlarge) |
 | Chromosome Structure
(click to enlarge) |
The two 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.
After separation the two former chromatids are each called unreplicated chromosomes.
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