How Cohesin Cleavage Triggers Anaphase

Cohesion keeps duplicated DNA paired

After DNA replication, the two copies of a chromosome remain paired as sister chromatids. Cohesin protein complexes provide the linkage that resists premature separation while chromosomes attach to the mitotic spindle.

At metaphase, sister kinetochores should be connected to opposite spindle poles. This bipolar attachment generates the geometry needed to distribute one copy to each future daughter cell.

The checkpoint protects the transition

The spindle-assembly checkpoint delays anaphase while kinetochores remain unattached or improperly attached. When the attachment conditions are satisfied, the anaphase-promoting complex/cyclosome (APC/C) promotes destruction of securin, an inhibitor of the protease separase.

Freed separase cleaves a cohesin subunit that maintains the final chromatid linkage. Removing that molecular restraint lets spindle forces move the sisters toward opposite poles. The former chromatids are then described as daughter chromosomes.

Why this switch is effectively one-way

Proteolysis makes the transition difficult to reverse. Once securin and the relevant cohesion are destroyed, the cell cannot simply restore the metaphase arrangement by reversing a binding reaction. This irreversibility helps coordinate chromosome movement with later mitotic exit.

Mitosis and meiosis use cohesion differently

Mitosis releases sister chromatids in one division. Meiosis I instead separates homologous chromosomes while protecting centromeric sister cohesion; that remaining cohesion is released for sister separation in meiosis II. The shared molecular theme is controlled cohesion, but the timing and chromosome partners differ.

Why timing matters

If cohesion is lost too early, chromosomes can segregate incorrectly. If it is not removed at the proper time, chromatid separation fails. Accurate anaphase therefore depends on both mechanical spindle attachment and biochemical permission to cut the linkage.