Reproducibility of X-Ray Survival Curves for Yeast Cells. ∗

Abstract

Summary

Accurate and reproducible X-ray-survival curves were obtained for both haploid and diploid yeast under carefully controlled conditions. The haploid curve is exponential down to survivals of 10⁻⁴ or lower for old cultures; the diploid curve is sigmoid and a good fit can be obtained by use of the proper multi-target model. The slope of the haploid survival curve (the radiosensitivity) depends on the culture age, young cultures being more sensitive. The haploid curves exhibit “tails” at low survival levels; the position of the tail is also dependent on the culture age, young cultures showing the tail at higher survival levels. The goodness of fit of the experimental points of the haploid yeast survival curves to an exponential curve is evidence that the rate-limiting inactivation step is first order as postulated in the diffusion model of Zirkle and Tobias.