The analysis of single-strand breaks in E. coli using a curve-fitting procedure.

A curve-fitting technique has been used to analyse the activity profiles occurring when the tritium labelled DNA of irradiated E. coli is sedimented in an alkaline sucrose gradient. The fitted curve is a modification of an expression derived elsewhere, which assumes that DNA of unit length undergoes scissions at random positions on the DNA strand. The theoretical curves fit the data well, even after considerable repair or excision of the radiation damage has been effected by cellular enzymes. The profile resulting from sedimentation of unirradiated DNA may be fitted with the same theoretical expression provided that the velocity of centrifugation is not too high. The number of single-strand breaks derived from the analysis is almost unaffected by the presence of extraneous activity at the top or bottom of the sucrose gradient. The method is therefore more reliable than many other methods used to analyse sedimentation data.