Final answer:
During the spindle assembly checkpoint in cell division, the APC/C ubiquitylates and promotes the proteolytic degradation of securin. This allows the activation of separase and the subsequent separation of sister chromatids to opposite poles. The spindle checkpoint ensures proper chromosome segregation, and failure may lead to cell cycle arrest or apoptosis.
Step-by-step explanation:
During the spindle assembly checkpoint in cell division, anaphase promoting complex/cyclosome (APC/C) ubiquitylates and promotes the proteolytic degradation of securin, which is an inhibitor of separase. Once securin is degraded, separase is activated and it cleaves the cohesin protein complexes that hold sister chromatids together. This separation allows the sister chromatids to be pulled apart to opposite poles of the cell, ensuring proper segregation of chromosomes.
The M checkpoint, also known as the spindle checkpoint, monitors several events: protein phosphorylation leading to chromatin condensation, breakdown of the nuclear envelope, spindle fiber formation, and the tension on kinetochores from spindle fibers. When sufficient tension is achieved indicating all kinetochores are correctly attached, the APC/C is activated leading to the degradation of securin, allowing the cell cycle to proceed from metaphase to anaphase. Failure of this checkpoint can result in cell cycle arrest, preventing division until errors are corrected, or if the cell is unable to correct the errors, triggering apoptosis to prevent genetic abnormalities.
Regulatory proteins like Rb, p53, and p21 also play significant roles in other checkpoints of the cell cycle. p53, in particular, can induce cell cycle arrest or apoptosis in the presence of damaged DNA to maintain genomic integrity.