Practice question

Accurate chromosome segregation requires that all kinetochores achieve amphitelic attachment to spindle microtubules from opposite poles and generate tension. Unattached or improperly attached kinetochores generate wait signal that delays onset of anaphase to prevent mis-segregation. This delay is enforced by mitotic checkpoint complex, abbreviated MCC, composed of Mad2, Bub3, BubR1 (Mad3 in yeast), and Cdc20. At unattached kinetochores, scaffold protein Knl1 is phosphorylated by Mps1, recruiting Bub1-Bub3 and Mad1-Mad2 complexes that catalyze conformational conversion of open Mad2 to closed form that sequesters Cdc20, essential co-activator of anaphase-promoting complex/cyclosome (APC/C). MCC-bound Cdc20 cannot activate APC/C, preventing ubiquitination and degradation of securin and cyclin B, thereby keeping separase inactive and CDK1-cyclin B high. Once all kinetochores attach and tension is established, MCC production ceases, existing complexes are disassembled by p31comet and TRIP13, freeing Cdc20 to activate APC/C and drive anaphase onset and mitotic exit. The inhibitory signal is generated catalytically, with single unattached kinetochore producing multiple MCC complexes through Mad1-Mad2 template, allowing amplification and sustained APC/C inhibition throughout cell, illustrating how local attachment defect generates global cell cycle arrest via diffusible inhibitor and kinetochore-based signaling platform.