Final answer:
The M checkpoint, also known as the spindle checkpoint, ensures proper attachment of chromosomes to the spindle fibers during metaphase of mitosis, preventing chromosome nondisjunction.
Step-by-step explanation:
In the M checkpoint, a chromosome improperly attached to the mitotic spindle is checked. This checkpoint is crucial as it occurs near the end of the metaphase stage of mitosis, also referred to as karyokinesis. The M checkpoint, also known as the spindle checkpoint, ensures that all of the sister chromatids are correctly attached to the spindle microtubules through their kinetochores. The cycle is halted until it is confirmed that the kinetochores are firmly anchored to at least two spindle fibers arising from opposite poles of the cell, preventing the nondisjunction of chromosomes.