Effect of Radiation on DNA Synthesis in Mammalian Cells

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

Nucleic acid synthesis in x-irradiated thymocytes

The rate of incorporation of thymidine into the deoxyribonucleic acid (DNA) of rat thymocytes in vitro was not immediately affected by low doses of X-radiation, but became progressively more inhibited as the irradiated cells were incubated at 37 °C for periods of up to 6 hours. Ribonucleic acid (RNA) synthesis deteriorated almost at the same rate as DNA synthesis after X-irradiation in vitro, but protein synthesis was slightly more resistant. Addition of 10–50 mM nicotinamide to the irradiated cell suspensions tended to retard the development of this inhibition, particularly at low temperatures, but high concentrations of nicotinamide were also toxic to these cells. The inhibition of DNA synthesis by X-irradiation appeared to follow a qualitatively similar pattern in thymus, spleen, and regenerating liver in vivo.In addition to the inhibition resulting from degenerative processes in the irradiated cells, DNA synthesis in vitro was directly inhibited by 20–30 kr X-radiation. The effects of radiation on DNA synthesis paralleled to some extent its effects on the gel-forming capacity of the deoxyribonucleoprotein from the thymus cells. It is suggested that the normal synthesis of both RNA and DNA in thymocytes depends on the integrity of the deoxyribonucleoprotein.