Date Approved


Date Posted


Degree Type

Open Access Senior Honors Thesis

Department or School


First Advisor

Anne Casper

Second Advisor

Aaron Liepman


Common fragile sites (CFSs) are present in a number of species and are considered a normal structural feature of the eukaryotic chromosome. CFSs are regions of genetic instability prone to breaking under DNA replication stress. Cancer is a possible result of DNA breaks that lead to a loss of heterozygosity (LOR) in the cell. Mitotic homologous recombination is a method of DNA repair following a break from DNA instability. Our experiments investigated the role of mitotic recombination in the development of LOH. Replication stress was induced in Saccharomyces cerevisiae through the use of the GALl/lO promoter; this modified promoter is dependent on the galactose concentration in the media for the expression of polymerase alpha. We examined a strain with an intact fragile site (FS2 of the yeast chromosome III) as well as a control strain with FS2 interrupted by a NAT gene. We hypothesized that DNA breaks at FS2 during mitosis, under conditions of replication stress, would be repaired by homologous recombination. Higher frequencies of mitotic recombination events leading to LOH were observed for the strain with an intact FS2 region, when subjected to replication stress and compared with the control strain in conditions of unstressed growth (9.83 fold increase). Our data adds further support to LOH resulting from mitotic recombination following the formation of fragile site breaks as the experimental strain under replication stress had higher frequencies of mitotic recombination events than the control strain.