The rejoining of single-strand breaks induced by γ irradiation in plasmid DNA under different scavenging conditions is described using human cell extracts. As the scavenging capacity of the irradiated solution increases from 1.5× 107 to <tex-math>$3\times 10^{8}\ {\rm s}^{-1}$</tex-math> using Tris-HCl as a scavenger, the ratio of single- to double-strand breaks is reduced from ≈70:1 to 40:1. After irradiation, a proportion of DNA molecules have no initial strand breaks but contain damage that is converted to strand breaks when incubated either at 37°C or in the presence of cellular extract. Repair of damage by the extracts is dependent upon the scavenging capacity of the irradiated solution. Optimal rejoining is observed when the scavenging capacity is <latex>$<1.5\times 10^{7}\ {\rm s}^{-1}$</latex>, and results in the repair of some initial strand breaks. As the scavenging capacity increases to <tex-math>$3\times 10^{8}\ {\rm s}^{-1}$</tex-math> the proportion of breaks repaired is significantly reduced. The relative increase in the yield of double-strand breaks and reduced repairability of single-strand breaks at a scavenging capacity of <tex-math>$3\times 10^{8}\ {\rm s}^{-1}$</tex-math> is consistent with the concept that the severity of damage increases upon increasing the scavenger concentration.

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