Sister chromatids are the two identical nucleoprotein strands of a single replicated eukaryotic chromosome.
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Chromosomes are paired, moreover,
each replicated chromosome is itself
composed of a pair of chromatids.
(click to enlarge)
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Chromosome Structure
(click to enlarge)
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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 >>).
Therefore — except after crossing-over during meiosis — 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.
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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A single replicated chromosome has two
chromatids. One chromatid is shown in
yellow, the other in blue.
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