The Formation of Single-Strand Breaks in Intracellular DNA by X-Rays

The number of x-ray induced DNA single-strand breaks under physiological conditions was determined by measuring production of the first break in closed covalent DNA molecules of bacteriophage λ in immune host bacteria. Under conditions of little repair, the anoxic rate of DNA single-strand breakage is 1.3× 10⁻⁶ DNA single-strand breaks per rad per 106 daltons. Under full oxygenation closed covalent DNA is 3.3 times more sensitive to strand breakage, the yield being 4.4× 10⁻⁶ breaks/rad/10⁶ daltons. Oxygen-promoted strand breaks formed at ice temperature are repaired almost instantaneously at 38°C, and probably represent a harmless lesion.