Final answer:
The cell cycle is halted at the M checkpoint if the chromosomes are not properly attached to the spindle microtubules.
Step-by-step explanation:
The M checkpoint, also known as the spindle checkpoint, is the checkpoint at which the cell cycle is halted if the chromosomes are not properly attached to the spindle microtubules. This checkpoint occurs near the end of the metaphase stage of mitosis. It ensures that all the sister chromatids are correctly attached to the spindle fibers before proceeding to anaphase.