Final answer:
The spindle-assembly checkpoint, or M checkpoint, monitors the proper attachment of chromosomes to the spindle fibers to ensure correct chromosome segregation during cell division. It prevents progression to anaphase until all chromosomes are correctly attached, safeguarding against chromosomal missegregation.
Step-by-step explanation:
The spindle-assembly checkpoint, also known as the M checkpoint, is crucial for ensuring that cells divide correctly. It monitors chromosome attachment to spindle fibers to make sure that each kinetochore is properly attached to spindle microtubules arising from opposite poles of the cell. This checkpoint prevents the cell cycle from proceeding to anaphase until all the chromosomes are accurately aligned and attached, ensuring that each daughter cell will receive the correct number of chromosomes during cell division.
At the M checkpoint, tension in the spindle apparatus tugs at the kinetochores holding the duplicated chromatids together. Once the necessary tension threshold is reached, signified by proper attachment of the kinetochores to the spindle fibers, the cell is allowed to proceed to the next phase. This mechanism serves as a critical safeguard against chromosomal missegregation and genetic instability, which could lead to conditions such as nondisjunction or aneuploidy if left unchecked.