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Sister Chromatids and the Centromere

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After DNA replication, a chromosome consists of two identical sister chromatids joined by cohesin proteins at a constricted region called the centromere.

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A replicated chromosome's two sister chromatids remain joined at the centromere until a later division stage pulls them apart.

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Sister chromatids are identical copies of one chromosome, not separate chromosomes — they count separately only after splitting apart.