Cohesin and its regulation: on the logic of X-shaped chromosomes

The X shape of chromosomes is one of the iconic images in biology. Cohesin actually connects the sister chromatids along their entire length, from S phase until mitosis. Then, cohesin's antagonist Wapl allows the separation of chromosome arms by opening a DNA exit gate in cohesin rings. Centromeres are protected against this removal activity, resulting in the X shape of mitotic chromosomes. The destruction of the remaining centromeric cohesin by Separase triggers chromosome segregation. We review the two-phase regulation of cohesin removal and discuss how this affects chromosome alignment and decatenation in mitosis and cohesin reloading in the next cell cycle.