<P> The MSH4 and MSH5 proteins form a hetero - oligomeric structure (heterodimer) in S. cerevisiae and humans . In S. cerevisiae, MSH4 and MSH5 act specifically to facilitate crossovers between homologous chromosomes during meiosis . The MSH4 / MSH5 complex binds and stabilizes double Holliday junctions and promotes their resolution into crossover products . An MSH4 hypomorphic (partially functional) mutant of S. cerevisiae showed a 30% genome - wide reduction in crossover numbers, and a large number of meioses with non-exchange chromosomes . Nevertheless, this mutant gave rise to spore viability patterns suggesting that segregation of non-exchange chromosomes occurred efficiently . Thus it appears that CO recombination facilitates proper chromosome segregation during meiosis in S. cerevisiae, but it is not essential . </P> <P> The fission yeast Schizosaccharomyces pombe has the ability to segregate homologous chromosomes in the absence of meiotic recombination (achiasmate segregation). This ability depends on the microtubule motor dynein that regulates the movement of chromosomes to the poles of the meiotic spindle . </P>

When do the x and y chromosomes segregate