Degradation of DNA in haemophilus influenzae cells after X-ray irradiation I. Experimental results.

Sequential measurements of DNA in Haemophilus influenzae cells after X-ray irradiation show rapid initial degradation of DNA followed by a plateau after about 40 min at normal growth conditions. Both the initial rate and final amount of degradation increase with radiation exposure. Degradation is somewhat greater in stationary-phase than in log-phase cells, but colony-forming ability (CFA) is independent of cell stage. Distributions of single-strand lengths of DNA in unirradiated or irradiated cases, as measured by alkaline sucrose gradient techniques, are neither monodispersive nor random, and possible causes for nonrandomness are discussed. The energy dissipated in the DNA is estimated as 40-50 eV per single-strand break for log-phase cells. The fractions of initial DNA remaining in heavily irradiated cells after long incubation are much greater than either the residual CFA or the number of DNA strands free of breaks. Hence, we conclude that cellular degradation of DNA, after exposure to ionizing radiation, cannot be explained quantitatively or qualitatively by simple correlations to these measures of cellular damage, but rather requires a more complex theory.