Spontaneous mutation of some genes was studied in haploid adenine and leucine auxotrophic yeast Saccharomyces. It was shown that a decrease in the amount of adenine (from 500 to 0 mg l-1) or leucine (from 300 to 0.3 mg l-1) in the medium, simultaneously with the transition from repression to derepression of the biosynthesis of these metabolites, resulted in a 15- to 150-fold increase in the reversion rate of genes ade 2 and leu2, respectively, for different strains. At the same time the mutation rate of suppressor genes varied relatively little (up to five-fold), and that of gene lys did not change at all. It was also demonstrated (on gene leu2) that the mutation rate is determined by the composition of the nutrient medium at the time of the S-phase of the cell cycle and it does not depend on the cultivation conditions during the presynthetic period. We discuss the hypothesis that derepressed genes mutate with a significantly higher rate than genes in the repressed state.

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The debate as to whether a low dose of radiation is "harmful, innocuous, or beneficial" is continuing. The debate stems from the fact that lethal and genetic effects have not been recorded in the range of low doses in classical radiobiology; such effects are almost always concealed in the physiological corridor or the interval of mean values.The distinctive features of the mechanisms by which cell structures are affected at a low radiation dose have now been explained. Physical-chemical and biochemical research has shown that conformational genome restructuring is observed under low-dose irradiation and the properties of the intracellular medium change. In view of this, the sensitivity of biomacromolecules, cells, and organisms to the effect of other damaging factors changes [1]. The changes can be directly or inversely dependent on the dose. An example of radiation damage to massive structures in cells is the change in the concentration of free radicals and the number of antioxidants in the system regulating the metabolism of membranes (Fig. 1). The changes in the intracellular medium and the conformational genome changes probably cause the observed destabilization of the genotype [2, 3].A low radiation dose has been shown to stimulate cell division [4], cause metabolic shifts that are incompatible with the vital activity of the cell, interphase death [5], and differentiating effects [6]. In some cases, the efficiency observed is higher for a low dose [7], the explanation for this being an increased radiosensitivity in that range of doses.Analysis of the given data about the changes caused in the cell by a low radiation dose shows that its adaptive-compensating functions disappear or are suppressed. This pertains primarily to the adaptive response of cells to irradiation. In the case of preliminary low-dose irradiation the adaptive response disappears and the cell is even sensitized. Interestingly, this