Abstract
Numerous studies have demonstrated the existence of DNA repair systems which function to preserve the basic integrity of genetic material. These repair systems are of two basic classes: the first class contains a repair system which functions only in the presence of light (photoenzymatic repair); the second class includes those systems which function optimally in the dark (excision and recombinational repair).| Photoenzymatic repair is known to be specific for chromosomal lesions which have been induced by ultraviolet light. In contrast, the dark repair systems are error-correcting mechanisms which alleviate the damaging effects of chromosomal lesions produced by a wide variety of agents, including X-radiation, nitrosoguanidine, and mitomycin C. In addition, it has been reported that mutants deficient in either excision or recombinational repair produce significant numbers of non-viable cells. These observations support the role of DNA dark repair systems in the general maintenance of chromosomal DNA.