Sister chromatids act like homologous chromosomes when the meiotic protein Pds5 is missing, Jin et al. show. The work reveals that the protein ensures that only homologous chromosomes pair up during meiosis.
Meiosis ends with chromosome separation, but it starts with togetherness. Homologous chromosomes synapse, pairing up with help from the synaptonemal complex. Sister chromatids also get close, as the cohesin complex fuses them along their length. One protein that helps sisters stay together is Pds5. But probing Pds5's function has been tricky because loss of the protein is lethal. Jin et al. were able to engineer yeast cells that only switch off Pds5 during meiosis.
Plenty went wrong in the cells, the researchers found. The chromosomes were abnormally short and dense, meiosis stalled during prophase I, and homologous chromosomes didn't synapse. Although the cells created the double-stranded chromosome breaks that allow crossing over, they didn't repair the fractures.
But the most striking defect was that sister chromatids attempted to synapse—the synaptonemal complex formed between sister chromatids instead of between homologues. Sister chromatids are closer together than are homologues, so the cell presumably has to prevent them from synapsing so that homologues can line up. The researchers found that Pds5 works by suppressing Rec8, a cohesin component. They now want to tease out the rest of the molecular mechanism.