Sister chromatid cohesion is mediated by individual cohesin complexes

Eukaryotic genomes are organized by loop extrusion and sister chromatid
cohesion, both mediated by the multimeric cohesin protein complex.
Understanding how cohesin holds sister DNAs together, and how loss of
cohesion causes age-related infertility in females, requires knowledge as
to cohesin’s stoichiometry in vivo. Using quantitative super-resolution
imaging, we identify two discrete populations of chromatin-bound cohesin
in post-replicative human cells. While most complexes appear dimeric,
cohesin localized to sites of sister chromatid cohesion and associated
with Sororin is exclusively monomeric. The monomeric stoichiometry of
Sororin:cohesin complexes demonstrates that sister chromatid cohesion is
conferred by individual cohesin rings, a key prediction of the proposal
that cohesion arises from their co-entrapment of sister DNAs.