Comparison of the x-ray responses of cells of isogenic haploid, diploid, and hexaploid strains of Saccharomyces cerevisiae showed that the early budding cells of diploids are significantly more resistant than those of haploids, which are in turn more resistant than those of hexaploids. Studies of synchronous cultures showed that, although with all strains the fraction surviving a single dose oscillates between extreme values during the cell cycle, the haploids showed the longest radioresistant phase and the hexaploids the shortest, and that increase in ploidy diminishes the relative increase in resistance achieved during prebudding and early budding. In view of evidence that budding cell resistance results from repair and the evidence favoring an increasing role with increased ploidy of dominant (relative to recessive) lethal damage in the x-ray inactivation of yeast, it is proposed that budding cell repair is more effective upon recessive than upon dominant lethal damage.

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