Biology / Unit 4 Bio 0883 short lesson, about 20 seconds
The M (Spindle) Checkpoint
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The idea
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.
In practice
One unattached kinetochore is enough to hold the whole cell at the M checkpoint, delaying anaphase for every chromosome.
Where people slip
The M checkpoint exists because chromatid separation cannot be undone — the cycle waits until attachment is verified.
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