Cell-cycle dependent changes in sensitivity to γ-rays in synchronously dividing yeast culture

Abstract

Changes in survival of yeast cells following γ-irradiation at different stages of the cell cycle were studied using a well synchronized culture. Maximum radioresistance occurs at the end of the S phase. Maximum radiosensitivity is observed just before entry into the S phase. The high degree of synchrony obtained allows more precise measurement of the extent of survival changes than has been achieved until now with partially synchronized cultures. Indeed, after a 60 krad irradiation we find a 100 % survival for cells which have just finished the S phase of the first cell cycle, against a 2 % survival for cells which are ready to enter the S phase of the second cell cycle. As the culture desynchronizes through successive cell cycles we have been able to follow the way in which survival curves are modified. We can extrapolate that with a perfectly synchronized culture the survival of ‘early S’ cells to a 60 krad irradiation would not be 2 % but 0.01 %. The high radioresistance observed at the end of S phase can hardly be explained simply in terms of DNA target or accumulation of radioprotectors. More likely the end of the S phase is a favourable stage for repair processes, at which time two genomes are able to recombine.