What is checked at the spindle checkpoint?

The spindle checkpoint is a crucial control mechanism that ensures the proper segregation of chromosomes during cell division, specifically during the metaphase stage of mitosis. It checks whether each chromosome is correctly attached to the spindle apparatus through their kinetochores, which are protein structures at the chromosomes' centromeres. Proper attachment is essential for ensuring that all chromosomes are pulled apart accurately to each daughter cell.

If any chromosome is not correctly attached, the spindle checkpoint will prevent the onset of anaphase, thereby halting cell division until the issue is resolved. This is vital for maintaining chromosomal stability and preventing conditions like aneuploidy, where daughter cells end up with an incorrect number of chromosomes, which can lead to cancer or other genetic disorders.

The focus on chromosome attachment is critical; while chromosome number consistency is important for genomic stability, it is primarily evaluated at other checkpoints earlier in the cell cycle, such as during the G1 and G2 phases. Cell size adequacy and nutritional status impact overall cell health and division but are not the primary focus of the spindle checkpoint itself.