During metaphase, the spindle fibers (or "microtubules") attach themselves to the centromere of each chromosome. Specifically, the connection is to specialized regions called kinetochores within the centromeres. Each chromatid has one kinetochore.
Now, during anaphase, the two sister chromatids of each chromosome are pulled apart by the spindle and dragged by their kinetochores toward opposite poles of the cell (i.e., toward the opposite centrosomes). The movement results from a shortening of the spindle microtubules. Each chromosome† is pulled along by its centromere. Formally, this phase begins when the duplicated centromeres of each pair of sister chromatids separates, and the resulting "daughter chromosomes" begin moving toward the poles. As the separated chromosomes move away from each other toward the poles, the cell elongates and the poles themselves move further apart.
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Compare this simplified diagram with the
photomicrograph at left.
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†Note: The term chromatid is used only to
refer
to one of the two united halves of a replicated
chromosome. Once the sister chromatids move
apart, each is called a daughter chromosome
or unreplicated chromosome.
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