AN IMPROVED TECHNIQUE OF STRAND BREAK ANALYSIS FOR ISODISPERSE DNA

A technique for determining the double strand break (dsb) yield in irradiated isodisperse DNA using the molecular length profile of the damaged DNA is described. The technique employs visualization of the irradiated DNA fragments on the EM 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. 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 phage T7 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. There is no evidence of bias caused by preferential losses in any part of the length profile.