Ultraviolet and X-irradiation produce many of the same effects on rat thymocytes in vitro. Exposure of the cells to low doses results in a latent lethal effect, which requires incubation at 37 °C for its expression, and in an apparent increase in the rate at which deoxyribonucleoprotein from the cells dissolves to form a gel in 2 M NaCl. As the doses are increased, a decrease in the viscosity of the deoxyribonucleoprotein gel, an immediate "death" and swelling of the cells, and an immediate destruction of nucleotide bases become evident. For either radiation, doses which do not appear to have any immediate effect on the intact cell lead to loss of soluble materials (potassium ion, ribonucleotides) from the cells after incubation at 37 °C for several hours. The amount of ultraviolet energy required to produce most of the above effects is nearly 1000 times greater than the amount of X-ray energy required to produce the same effect.Three differences between the effects of ultraviolet and X-irradiation were observed: Ultraviolet light was relatively inefficient in breaking down the long deoxyribonucleoprotein chains but appeared to destroy hydrogen-bonding in the native nucleic acid structure at the same time as it destroyed the nucleotide basis. Moreover, the lethal effects of low doses of ultraviolet light on thymus cells require a longer time to develop than do the effects of X-irradiation.Thymocytes can also be killed by high intensities of visible light, particularly in the presence of photoreducible dyes. However, visible light produces little or no latent damage to the cells, nor was any evidence of nucleic acid damage observed.
Two-dimensional gel analysis of rolling circle replication in the presence and absence of bacteriophage T4 primase