Sister chromatid cohesion mediated by cohesin is essential for mitotic fidelity. It counteracts spindle forces to prevent premature chromatid individualization and random genome segregation. However, it is unclear what effects a partial decline of cohesin may have on chromosome organization. In this study, we provide a quantitative analysis of cohesin decay by inducing acute removal of defined amounts of cohesin from metaphase-arrested chromosomes. We demonstrate that sister chromatid cohesion is very resistant to cohesin loss as chromatid disjunction is only observed when chromosomes lose >80% of bound cohesin. Removal close to this threshold leads to chromosomes that are still cohered but display compromised chromosome alignment and unstable spindle attachments. Partial cohesin decay leads to increased duration of mitosis and susceptibility to errors in chromosome segregation. We propose that high cohesin density ensures centromeric chromatin rigidity necessary to maintain a force balance with the mitotic spindle. Partial cohesin loss may lead to chromosome segregation errors even when sister chromatid cohesion is fulfilled.
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