Regulation of sister chromatid separation at the metaphase-anaphase transition (Homo sapiens)

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In prometaphase, highly condensed chromosomes establish bipolar attachments to the mitotic spindle. Unattached or mal-orientated chromosomes generate a signal to delay the onset of anaphase until all pairs of sister chromatids are properly aligned on the metaphase plate. This signal is transduced by a relay of spindle-checkpoint proteins, including MAD1, MAD2, BUB1, BUBR1, BUB3 and centrosome protein E (CENPE), which inhibits cell division cycle 20 (CDC20)-mediated activation of an E3 ubiquitin ligase, the anaphase promoting complex/cyclosome (APC/C). Following attachment and alignment of all the chromosomes at metaphase, the checkpoint signal is silenced and APC/C initiates the ubiquitin-dependent degradation of securin and activation of separase. Separase in turn cleaves a multiprotein complex termed cohesin, which creates physical links between sister chromatids to initiate anaphase. Adapted from Pino et al.