The M (Spindle) Checkpoint
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The M checkpoint, also called the spindle checkpoint, occurs near metaphase's end and blocks sister-chromatid separation until every kinetochore attaches to microtubules from opposite poles.
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One unattached kinetochore is enough to hold the whole cell at the M checkpoint, delaying anaphase for every chromosome.
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The M checkpoint exists because chromatid separation cannot be undone — the cycle waits until attachment is verified.
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