Characterization of genetic coding changes in bacteria produced by ionizing radiation and by the radioactive decay of incorporated 3 H-labelled compounds

Cosmic radiation and evolution of life on earth: roles of environment, adaptation and selection

The role of ionizing radiation in general, and cosmic radiation in particular, in the evolution of organisms on the earth by adaptation and natural selection is considered in a series of questions: (1) Are there times during the evolution of the earth and of life when genetic material could be exposed to heavy ion radiation? (2) Throughout the course of chemical and biological evolution on the earth, what fraction of environmental mutagenesis could be attributable to cosmic and/or solar ionizing radiation? (3) Is ionizing radiation an agent of adaptation or selection, or both? (4) What can the cladistics of the evolution of genetic repair tell us about the global history of genotoxic selection pressures? (5) How much genetic diversity can be attributed to the selection of radiation-damage repair processes?

The nature of mutants induced by ionising radiation in cultured hamster cells. I. Isolation and initial characterisation of spontaneous, ionising radiation-induced, and ethyl methanesulphonate-induced mutants resistant to 6-thioguanine

A large number of thioguanine (TG)-resistant mutants of V79-4 Chinese hamster cells was isolated from untreated cultures and from cultures exposed to gamma-rays, alpha particles or ethyl methanesulphonate (EMS). Selection conditions were chosen to optimise the survival of all types of TG-resistant mutant, and the isolation procedure ensured that each mutant originated independently of any other. Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) enzyme activity was measured in cell-free extracts of each mutant, and compared with repeat measurements made on the parental V79-4 cells and on a series of non-mutant clones. Ionising radiation-induced mutants were found to be mostly (or perhaps entirely) of the 'zero HGPRT activity' type, but about 20% of EMS-induced mutants and 50% of spontaneously occurring mutants had significant HGPRT activity. However, none of the TG-resistant mutants were found to lack activity of another X-chromosome-linked enzyme, glucose-6-phosphate dehydrogenase. Few mutants were found with visible X-chromosome changes, but the incidence of hyperploidy was higher among spontaneous mutants than in the parental line and the induced mutants. Isoelectric focussing of cell extracts from those mutants which retained some HGPRT activity revealed several with shifts in the isoelectric points for HGPRT enzyme activity.

Effect of HZE particles and space hadrons on bacteriophages

The effect of high energy (HZE) particles and high energy hadrons on T4Br+ bacteriophage was analyzed. The experiments were done in orbital flight, on high mountains, on an accelerator, and with an alpha particle source. We studied the survival rate of the bacteriophage, the mutation frequency, the mutation spectrum and the revertability under the action of chemical mutagens with a known mechanism of action on DNA. It was found that the biological efficiency of HZE particles and high energy hadrons is greater than that of gamma radiation. The spectra of mutations produced by these mutations and the mechanisms of their action are also different. These effects were local, because of the mode of interaction of the radiant energy with biological objects, and depended on the linear energy transfer (LET). The modes have now been experimentally defined.