Saturation of Repair
The deleterious effects of the exposure of cells, tissues or animals to exogenous radiations and chemicals are ameliorated by DNA repair processes. Many repair pathways are constitutive, and some, such as the SOS system in bacteria, are inducible while others, such as the UvrABC excision system in E. coli, are both constitutive and inducible (Friedberg, 1985). The ability of bacteria to adapt to low chronic doses of N-methyl-N-nitro-N-nitro-soguanine (MNNG) so as to become resistant to the cytotoxic or mutagenic effects of a large challenge dose is an important example illustrating that low chronic exposures not only may protect against subsequent high acute ones but also that extrapolation from high acute doses to low chronic ones may grossly overestimate the biological effects of the low chronic doses (Robins and Cairns, 1979). Another example is where low doses of nitrosamines protect rats against the carcinogenic effects of a later high dose (Montesano et al., 1980; Margison, 1982). In bacteria, chronic exposure to MNNG involves adaptation along separate pathways against mutagenic and cytotoxic agents.
KeywordsExcision Repair Xeroderma Pigmentosum Repair Rate Pyrimidine Dimer Skin Cancer Incidence
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