Additivity of X-Rays and Ultraviolet Light in the Inactivation of Haploid and Diploid Yeast

Abstract

Haploid and diploid yeast were inactivated (i.e., visible colony formation was inhibited) by exposure to 250 kv X-rays and to UV of 2537 A[degree]. It is shown that for both kinds of radiation the difference in sensitivity between the haploid and diploid can be simply explained by assuming that the haploid cell contains a number of sensitive sites, all of which are necessary to cell survival, and that the diploid contains a duplication of these sensitive sites such that either member of a pair of sites is sufficient to maintain cell function. Additivity experiments were performed in which cells were pretreated with one kind of radiation before survival curves were run with the alternate radiation. The 2 radiations showed a great degree of interaction in the diploid yeast but almost no interaction in the haploid. These results support the hypothesis that lessened sensitivity in the diploid is due to a replication of the sensitive sites of the haploid and indicate that, to an appreciable extent, the sensitive sites involved in X-ray and in UV inactivation are the same. No attempt has been made to relate the sensitive sites to morphological entities within the cell.