Final answer:
In mitosis, the spindle checkpoint delays the onset of anaphase if a chromosome is not properly aligned during metaphase.
Step-by-step explanation:
In mitosis, if a chromosome fails to become aligned properly at the metaphase, the mechanism that will delay the onset of anaphase is the spindle checkpoint. The M checkpoint, also known as the spindle checkpoint, determines whether all the sister chromatids are correctly attached to the spindle microtubules. Because the separation of sister chromatids during anaphase is an irreversible step, the checkpoint ensures that the kinetochores of each pair of sister chromatids are firmly anchored to at least two spindle fibers arising from opposite poles of the cell, preventing premature progression to anaphase.