ACTION SPECTRA FOR SINGLE-STRAND AND DOUBLE-STRAND BREAK INDUCTION INPLASMID DNA - STUDIES USING SYNCHROTRON-RADIATION

Ionizing radiations deposit a wide range of energies in and around DNA and this leads to a corresponding spectrum of complexity of the lesio ns induced. The relationships between the amount of energy deposited a nd the yields and types of damage induced are important in modelling t he physical and chemical stages of radiation effect and linking them t o biological outcome. To study these relationships experimentally, pla smids were mounted as a monolayer and exposed in vacuum to near-monoen ergetic photons from the Daresbury Synchrotron. After irradiation, the DNA was washed off and assayed for single-(ssb) and double-strand bre aks (dsb) using agarose gel electrophoresis. Dose-effect relationships for ssb and dsb induction were obtained at various energies in the ra nge 8-25 eV. The initial responses in the low-dose region allowed dama ge yields to be estimated. However, a common feature is that the respo nses Showed energy-dependent plateaus at higher doses as if a fraction of the DNA were shielded. Various measures were taken both to minimiz e and to correct for this effect. The data appear to show that the yie lds of ssb and dsb increase only slowly with photon energies >10 eV, w ith a suggestion of similar threshold energies for both lesions. In th e energy range covered, the yield of ssb is 12-20-fold greater than th at of dsb. The data indicate that ssb and dsb may have a common precur sor in this system. Earlier work with low-energy electrons showed that at 25eV ssb were induced but no dsb were detected.