Amon and her colleagues found that phosphorylation was not the only process essential for cohesion removal. Recombination ?an exchange of DNA between chromosomes that promotes genetic diversity ?was also needed for the initial removal of cohesin from the chromosome arms, they found.
In meiotic recombination, after each member of a chromosome pair has replicated to produce identical sister chromotids in the initial stage of meiosis, the chromosomes exchange arm segments. Only after this exchange, or recombination, do the cells proceed to the second stage of meiosis -- dividing without chromosome replication to produce haploid sperm or egg cells. Recombination is essential for cohesins to get removed from chromosome arms before they are removed from centromeres, Amon said.
"For a long time, people did not think that recombination played any role in establishing the step-wise cohesin loss pattern," she said. "But our experiments showed that recombination is absolutely essential to remove cohesins from chromosome arms during the initial meiotic stages, and if you don't have recombination that does not happen properly."
Amon also said that the Nature paper represents a significant advance in understanding the enzyme that phosphorylates Rec8, named Polo kinase, because it more accurately identified the specific sites on the Rec8 subunit that Polo phosphorylates. Firm identification of such sites will enable more precise studies of the mechanism of function of Rec8, and will also enable scientists to identify other protein targets of Polo kinase, she said.
Amon emphasized that the discovery of the importance of phosphorylation and recombination is only the beginning of understanding the intricate, critical process
Source:Howard Hughes Medical Institute