The Relationship between the Rate of DNA Synthesis and Its Inhibition by Ultraviolet Light in Mammalian Cells

Abstract

The rate of 3HTdr [3H-thymidine] incorporation into DNA varied with temperature according to a linear Arrhenius relationship between 4[degree] and 40 [degree]C, with an activation energy of 25.8 kcal/mole. After irradiation with moderate doses only a small fraction of the genome completed replication, and DNA synthesis stopped within 2 hours. Dose-response curves for the inhibition of DNA synthesis at temperatures of 25[degree], 31[degree] and 37 [degree]C were all of the biphasic or multiphasic type, and the sensitivity to irradiation was higher when the rate of DNA synthesis was higher. The temperature during irradiation was irrelevant, and only the temperature after irradiation had any effect on the observed inhibition. The observations were interpreted in terms of a model in which blocks to replication are introduced into DNA by irradiation and replication ceases when the first block is reached. Simple calculations indicate that pyrimidine dimers may not be effective blocks in mammalian cells. The photoproducts that block replication are formed by UV with about 10% of the efficiency of dimer formation. An increase in the fraction of cells incorporating 3HTdr after irradiation was observed in the human cell strain, but not the mouse strain; this may be due either to DNA repair or to abnormal DNA synthesis.