Final answer:
The M checkpoint occurs near the end of the metaphase stage of mitosis and ensures that sister chromatids are correctly attached to the spindle microtubules. If chromosomes are not attached to the spindle, the M checkpoint will inhibit the anaphase promoting complex (APC).
Step-by-step explanation:
The M checkpoint, also known as the spindle checkpoint, occurs near the end of the metaphase stage of mitosis. It determines whether all the sister chromatids are correctly attached to the spindle microtubules. If chromosomes are not yet attached to the spindle, they will trigger the M checkpoint and inhibit the anaphase promoting complex (APC). The M checkpoint ensures that the separation of sister chromatids during anaphase does not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to the spindle fibers.