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The spindle checkpoint blocks entry into anaphase until all chromosomes are properly attached to the mitotic spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles. Only this pattern of attachment will ensure that each daughter cell receives one copy of the chromosome.
The spindle checkpoint is an active signal produced by improperly attached kinetochores. Unattached kinetochores trigger the spindle checkpoint. When sister kinetochores are properly attached to opposite spindle poles, forces in the mitotic spindle generate tension at the kinetochores. Kinetochores that are attached to the mitotic spindle but that are not under tension also trigger the spindle checkpoint. The mechanism by which kinetochores detect attachment and tension is unclear.
The spindle checkpoint blocks anaphase entry by inhibiting the anaphase-promoting complex.
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