Prometaphase and the Kinetochore
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Prometaphase is the stage where the nuclear envelope finishes fragmenting and each sister chromatid grows a kinetochore, a protein structure at the centromere that binds spindle microtubules.
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Spindle microtubules extending from opposite poles attach to a chromosome's two kinetochores, one set from each pole.
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A chromosome is not secured until both sister chromatids' kinetochores attach to microtubules from opposite poles.