DNS-Synthese nach Röntgenbestrahlung bei homozygoten Hefestämmen verschiedenen Ploidiegrades

Abstract

Since Saccharomyces cells have only a very small uptake of thymine or thymidine, the uptake of labelled samples of these compounds cannot be used to measure the rate of DNA synthesis. Instead two other methods were used for both X-irradiated and unirradiated cells of a series of homozygous isogenic Saccharomyces strains of different ploidy: 1. Yeast cells were incubated with adenine-8-C14, the labelled DNA was isolated and completely separated from RNA, and the specific radioactivity of the DNA was determined. The reduction of the specific radioactivity after irradiation as a proportion of the specific radioactivity of unirradiated control cells gave the X-ray induced inhibition in the rate of DNA synthesis. 2. The uptake of thymine-2-C14-5′-monophosphate, which is the only thymine compound incorporated into the DNA of Saccharomyces in any appreciable amount, was measured after various incubation times for irradiated and control cells. The results of the two methods are compared and the possibilities for their application are discussed. The dose effect curve for the rate of DNA synthesis after X-irradiation shows two phases: a steep slope at low doses with a shallow slope at high doses. For low doses the proportional decrease with X-irradiation in the rate of DNA synthesis is of the same order of magnitude as that of the colony forming ability, and is considerably greater than that of other metabolic processes such as protein and RNS synthesis, respiration or fermentation. The dependence of this proportional decrease on degree of ploidy is also similar to that dependence shown by colony forming ability and is again different from the dependence shown by RNA or protein synthesis. The reduction in DNA synthesis is greater the greater the time of incubation after irradiation.