Strand Breaks, Base Release, and Postirradiation Changes in DNA g-Irradiated in Dilute O 2 -Saturated Aqueous Solution

γ-Irradiation of DNA in dilute ${\rm O}_{2}\text{-Saturated}$ aqueous solution releases free bases and damaged bases from the macromolecule. The yields of these products were measured after column chromatographic separation. For double-stranded DNA the immediate yield of bases varies from G = 0.012 for cytosine to G = 0.033 for adenine. The yields of released bases increase with postirradiation time (the majority of the increase occurs in the first 2 hr) to yields in the range of G = 0.07 to ±0.012. Yields of two released damaged thymine radiation products from γ-irradiated ³ H thymine-labeled DNA also increased with postirradiation time. Strand breaks were measured in γ-irradiated single-stranded DNA; the initial yield G = 0.02 was low but increased with time to G = 0.07. No direct correlation between strand-break production and release of products of low molecular weight is possible. The findings are discussed in terms of damage to DNA in vivo and its enzymatic repair.