Separation of Sister Chromatids

Cohesin protein ring-complexes hold sister chromatids together at the centromere region during metaphase, preventing premature separation. Residual cohesin connections persist from prophase through metaphase to block aneuploidy risks in daughter cells. This cohesion is maintained until separase is activated at the metaphase-to-anaphase transition, when cohesin rings are cleaved.

The anaphase-promoting complex (APC/C) is activated by cyclin-Cdk phosphorylation and binds to Cdc20, forming an active complex. This active APC/C-Cdc20 ubiquitinates securin for degradation, releasing separase. Separase then cleaves cohesin rings, causing absolute loss of sister-chromatid cohesion and allowing chromatids to separate.

Separase is initially inhibited by securin and by cyclin-Cdk-mediated phosphorylation. When APC/C degrades securin and cyclins, Cdk activity decreases, allowing phosphatases to dephosphorylate separase. This dephosphorylation activates separase, enabling it to cleave cohesin and release sister chromatids for poleward movement.

The spindle assembly checkpoint prevents APC/C activation until each chromosome correctly aligns on the mitotic spindle. This checkpoint blocks premature ubiquitination of securin and cyclins, ensuring sister chromatids remain cohesed until proper attachment is confirmed. Deactivation of the checkpoint allows ordered progression to anaphase.

APC/C-Cdc20 ubiquitinates cyclins A and B, targeting them for proteasomal degradation. Cyclin B destruction removes Cdk1 activity, inactivating the cyclin-Cdk complex. Loss of Cdk activity allows phosphatases to dephosphorylate and activate separase, which then cleaves cohesin to permit chromatid separation.

APC/C-Cdc20 is active during metaphase-to-anaphase transition, degrading securin and cyclins to promote anaphase. As cyclin B levels drop, Cdk1 inactivates, allowing APC/C-Cdh1 to become active. APC/C-Cdh1 then facilitates mitotic exit and stabilizes G1 phase by preventing premature mitotic cyclin accumulation.

Once separase cleaves cohesin rings, sister chromatids lose cohesion and are no longer held together. Poleward forces generated along microtubules then pull the separated chromatids toward opposite spindle poles. This force-driven movement completes anaphase and positions chromatids for cytokinesis and daughter cell formation.