Practice question

Anaphase entry requires anaphase-promoting complex/cyclosome (APC/C) activated by co-activator Cdc20, which targets securin and cyclin B for proteasomal degradation via K11-linked polyubiquitination. Securin normally binds and inhibits separase, protease that cleaves cohesin subunit Rad21 holding sister chromatids together. When APC/C-Cdc20 destroys securin, liberated separase removes cohesin, allowing sister chromatids to separate toward opposite poles. If Cdc20 is absent, inhibited by MCC, or chemically blocked, APC/C remains inactive, securin and cyclin B persist, separase stays inhibited, cohesin remains intact, and sister chromatids continue to cohere at metaphase plate despite spindle forces. Chromosomes therefore fail to separate, producing metaphase arrest with high CDK1 activity preventing mitotic exit. Conversely, premature Cdc20 activation causes early separase release and aneuploidy. Depletion experiments in yeast and mammalian cells show non-disjunction and prolonged checkpoint activation upon Cdc20 knockdown, while overexpression drives tumorigenesis through chromosomal instability. Hence Cdc20 inhibition specifically blocks chromosome segregation step, not DNA replication. Ubiquitin ligase APC/C conjugates K11-linked chains via Ube2S elongating enzyme after initial ubiquitination by UbcH10, creating signals recognized by 26S proteasome for rapid degradation, and this sequential ubiquitination ensures irreversible commitment to anaphase only after checkpoint silencing, preventing reversal that would cause partial segregation and chromothripsis.