ON THE EXPERIMENTAL DISTINCTION BETWEEN SSBS AND DSBS IN CIRCULAR DNA

Purpose: To determine directly the minimal distance between two ssbs o n complementary strands in circular DNA that are not observed as a dsb by electrophoresis. Materials and methods: 3.2kbp DNAs with cohesive overhangs of various lengths were systematically generated by a newly devised method and electrophoresed in agarose slabs. Results: At 4 deg rees C, 3.2kbp DNA with cohesive overhangs larger than 6 bp migrated a s circular DNA. The minimal overhang size for the DNA to migrate as ci rcular DNA was larger at 25 degrees C. Whether the DNA migrated as a c ircular or a linear molecule depended also on the nucleotide sequence of its overhangs, most notably at the minimal size. Conclusions: The m inimal distance obtained in the present study agrees with the smaller values of previous indirect estimates. The dependence of the distance on experimental conditions suggests feasibility of obtaining the stagg er-size distribution of radiation-induced dsbs.