Evidence for the dominance of direct excitation of DNA in the formation of strand breaks in cells following irradiation.

The yields of radiation-induced strand breaks measured in a plasmid DNA irradiated as a 'dry film' are similar to those measured in SV40 DNA irradiated in a cellular environment (Roots et al. 1985). This suggests a common mechanism, namely direct excitation of the DNA rather than indirectly inflicted damage from radiation-induced water radicals. This result is discussed in terms of a recently proposed mechanism of excitation transfer in DNA following direct excitation by ionizing radiation.