An improved technique of strand break analysis for isodisperse DNA

Abstract

A technique for determining the double strand break (dsb) yield in irradiated iso-disperse DNA using the molecular length profile of the damaged DNA is described. The technique employs visualisation of the irradiated DNA fragments on the electron microscope followed by length measurement of a randomly selected sample. The range of damage index measurable (0.01–1 dsb/106 Dalton) is substantially greater than that accessible by techniques involving centrifugation. Moreover the use of the whole profile of molecular lengths in the irradiated sample allows errors in the determination to be assessed. Results of experiments with T7 bacteriophage DNA irradiated in dilute aqueous solution indicate that the observed molecular length profiles are in good agreement with those generated theoretically assuming random breakage. The techniques for fitting observed profiles by the method of maximum likelihood to those generated theoretically are described in detail in an appendix. There is no evidence of bias caused by preferential losses in any part of the length profile.