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FORMATION OF SINGLE AND DOUBLE STRAND BREAKS IN DNA ULTRAVIOLET IRRADIATED AT HIGH INTENSITY

The induction of single‐strand breaks (SSB) by two quantum processes in DNA is well established. We now report that biphotonic processes result in double‐strand breaks (DSB) as well. pUC19 and bacteriophage M13 RF DNA were irradiated using an excimer laser (248 nm) at intensities of 107, 109, 1010 and 1011 W/m2 and doses up to 30 kJ/m2. The proportion of DNA as supercoil, open circular, linear and short fragments was determined by gel electrophoresis. Linear molecules were noted at fluences where supercoiled DNA was still present. The random occurrence of independent SSB in proximity to each other on opposite strands (producing linear DNA) implies introduction of numerous SSB per molecule in the sample. If so, supercoiled DNA that has sustained no SSB should not be observed.