Final answer:
The spindle assembly checkpoint, occurring at the end of metaphase, halts anaphase initiation until all sister chromatids are properly aligned with spindle microtubules, relying on an inhibitory signal from improperly attached chromosomes.
Step-by-step explanation:
The spindle assembly checkpoint (M Checkpoint) is a critical part of mitosis, specifically at the end of the metaphase stage, which ensures that each sister chromatid is properly attached to the spindle microtubules before anaphase can commence. This checkpoint relies on an inhibitory signal that halts the process if the chromosomes are not correctly aligned. Inappropriately attached chromosomes won't generate the necessary tension at the kinetochores to satisfy the checkpoint. Once all chromatids are correctly attached and proper tension is achieved, the inhibitory signal is lifted, allowing the cell to progress into anaphase.
The M Checkpoint functions to prevent the separation of sister chromatids until the kinetochores of each pair are firmly anchored to at least two spindle fibers arising from opposite poles. This ensures that each daughter cell will receive the correct number of chromosomes upon cell division completion.