Faithful transmission of chromosomes during cell division is essential to the functioning of the eukaryotic cell. In humans, errors during chromosome segregation are correlated with diseases such as Down Syndrome and cancer. To elucidate the genes and processes required for accurate chromosome segregation, two types of genetic approaches using the model eukaryotic organism Saccharomyces cervisiae (baker’s yeast) are being implemented. Both approaches rely on an assay system in which the segregation of a non-essential yeast artificial chromosome (YAC) is monitored. The first approach begins with the isolation of mutants with defective chromosome segregation and seeks to identify the gene(s) responsible for producing the mutant phenotype. Four previously isolated YAC stability in mitosis mutants (ysm’s 22, 77, 83 and 84) were characterized as having a marked increase in YAC loss. To identify genes that suppress the YAC loss phenotype in these mutant strains, the ysm mutants were transformed with a yeast genomic plasmid library and screened for plasmids that suppress the mutant phenotype. The suppressor candidates were re-screened, and the specific genes responsible for the suppression are currently being determined. The second approach begins with a known mutation on chromosome segregation. Five genes of known function that are suspected to be important for accurate transmission of chromosome (MRC1, MRE11, MUS81, RAD9, and RAD27) are targeted in this analysis. Following deletion of these genes in a yeast strain containing a YAC, the loss rate of the YAC will be experimentally determined in the deletion mutants and compared to the loss rate in a wild-type strain. These investigations are expected to reveal genes important for the process of chromosome segregation
