Sites and structure of gamma radiation-induced DNA strand breaks

DNA fragments of defined sequence were used to study DNA strand breakage by gamma radiation in doses ranging from 2,500 to 20,000 rads. The amount of radiation-induced DNA breakage is linearly dose-dependent, and strand scission events occur uniformly at all nucleotide sites, regardless of sequence. The 5'-terminal group at points of breakage is a simple phosphoryl moiety. Two types of radiation-induced 3' termini were identified. One species of 3' terminus is a simple phosphoryl group. The other species of 3' terminus is neither a hydroxyl nor a phosphoryl group. A model for radiation-induced DNA strand scission that involves destruction of the deoxyribose moiety is presented. Similarities among DNA strand scission events created by gamma radiation, ferrous ion, doxorubicin, and bleomycin are discussed.