Effects on DNA synthesis of localized irradiation of cells in tissue culture by (i) a u.v. microbeam and (ii) an α-particle microbeam

Abstract

The effect on the rate of DNA synthesis of irradiating parts of single cells in tissue culture with (1) a microbeam of [alpha]-particles 6 [mu]m in diameter, and (2) a microbeam of ultra-violet light 3.5 [mu]m in diameter is described. The sites irradiated in both cases were (i) the nucleolus, (ii) in the nuclear sap, (iii) in the cytoplasm, (iv) in the nutrient medium close to but outside, 5 to 10 /xm away from, the cell. Rate of DNA synthesis was measured by means of quantitative H3-thymidine flash autoradiography. The results were confirmed by measuring the change in total DNA/cell by microspectro-photometric means. The nucleolus appears to have no specific role in DNA synthesis since inhibition of DNA synthesis by irradiation of the nucleolus does not differ from that produced by irradiation of an equal area of the nuclear sap. Irradiation of either the cytoplasm or the nutrient medium also produces inhibition of DNA synthesis. Equal absorbed amounts of energy per unit volume of either ultraviolet light or [alpha] -particles produced inhibitions of the same order of magnitude. Inhibition of DNA synthesis in all cases needs several hours to develop fully. It is suggested that the inhibition mechanism may depend on the formation of a radiation-produced inhibitory substance which diffuses rapidly throughout the cell. The delay in the full development of the inhibition may arise from the fact that the inhibitory substance acts so as to prevent the replenishment of one of the pools of essential compounds in the DNA synthetic chain. In the light of these comments, other workers'' results which support these hypotheses are considered.