Enhancement of gamma-ray induced mutation in barley seeds by inhibitors of the unscheduled DNA synthesis

Gamma-ray causes single-strand DNA break leading to the formation of phosphomonester groups and to the production of 3'-hydroxyl termini. Linear relationship was found between the number of induced single-strand break per gram of DNA and the radiation dose applied to the seeds. 5'-phosphoryl termini may originate from the oxidation of the C'-3 of deoxiribose and the subsequent splitting of the phosphoester bond. The rate of DNA synthesis in γ-irradiated seeds during the early stage of soaking was larger than that in no-irradiation seeds. In the first-leaf meristem cells of 30 kR-irradiated seeds, the nuclei expanding 2 to 4 times in size and containing fibrous dispersed chromatin increased significantly. The caffeine enhancement of the damage in the γ-irradiated seeds of barley was ascribed to the inhibition of repair or the rejoining of chromatid break. The potentiating effect of caffeine on the chromosome aberration induced by γ-irradiation took place only during the early stage of germination of irradiated seeds. EDTA enhanced radiation-induced chlorophyll mutation. It seems likely that the interference with the processes of repair replication leads to more mutations. (Yamashita, S.)