Cohesin doesn't shirk its duties during mammalian meiosis after all. Llano et al. reveal that the protein complex forms part of a structure that enables homologous chromosomes to pair up.
During mitosis, the cohesin complex holds sister chromatids together with a ring-like structure. During meiosis in yeast, cohesin has an additional function in forming axial elements, adhesive strips that help homologous chromosomes line up opposite one another. This juxtaposition enables homologues to swap genetic material through crossing over and to repair damaged DNA through homologous recombination. Yeast lacking one meiosis-specific component of cohesin, the kleisin REC8, can't construct axial elements. But what happens in mammals is unclear, probably because our genomes have a gene for a second meiotic kleisin. Mouse spermatocytes lacking either REC8 or the other kleisin, RAD21L, can generate axial elements, suggesting that cohesin isn't necessary for their construction.
To clarify cohesin's job in mammalian meiosis, Llano et al. crossed mice to produce animals that were missing REC8 and RAD21L. The mice appeared healthy. However, they were sterile, and their cohesin complexes were faulty, lacking two other components besides REC8 and RAD21L. Spermatocytes from the animals were missing axial elements, and their homologous chromosomes didn't pair up at all. Although cells from these mice formed the double-strand breaks that permit DNA repair, they couldn't mend these breaks because a key repair enzyme, RAD51, didn't arrive at the chromosomes. Overall, the researchers say, the results suggest that cohesin is necessary for axial element construction in mammals.