"Break copy" duplication: a model for chromosome fragment formation in Saccharomyces cerevisiae.

Introduction of a chromosome fragmentation vector (CFV) into the budding yeast Saccharomyces cerevisiae results in a targeted homologous recombination event that yields an independently segregating chromosome fragment (CF) and an alteration in the strain's karyotype. Fragmentation with an acentric CFV directed in a centromere-proximal orientation generates a CF that contains all sequences proximal to the targeting segment and results in loss of the endogenous targeted chromosome to yield a 2N-1 + CF karyotype. In contrast, fragmentation with a centric CFV directed in a centromere-distal orientation generates a CF that contains all sequences distal to the targeting segment and retention of the endogenous targeted chromosome to yield a 2N + CF karyotype. We have termed this phenomenon "break copy" duplication. Using yeast strains in which the centromere had been transposed to a new location, it was demonstrated that the centromere inhibited break copy duplication. These data suggest that CF formation is the product of an unscheduled DNA replication event initiated by the free end of the CFV and is analogous to a "half" double-strand break gap-repair reaction. We suggest that break copy duplication may have evolved as a mechanism for maintenance of ploidy following DNA breakage.