Handcuff for sisters: a new model for sister chromatid cohesion.

High fidelity chromosome transmission requires the pairing of sister chromatids by a group of conserved proteins called cohesins during S-phase. Sister chromatid cohesion is maintained until anaphase onset. Presently, there are two sets of models of cohesin complex popular in the cohesin biology field: one model predicts that single cohesin rings entrap both sister chromatids, and the other model proposes that cohesin complexes associate with each sister chromatid and become paired during DNA replication. It is the first model that currently predominate the field--in part because prior efforts failed to detect higher order cohesin-cohesin interactions. However, the static configuration and size limitation of the one ring embrace model are the major limitations of the embrace model, and cannot explain various functions of cohesin in DNA replication, DNA repair and gene expression. In a recent study published by Zhang et al. in the Journal of Cell Biology describes a two-ring handcuff model for the cohesin complex, and provides new information regarding how sister chromatid cohesion and separation are achieved in vertebrate cell systems.