Abstract
The wavelength dependence of some of the effects of ultraviolet radiation on the physicochemical properties of DNA of phage alpha irradiated in vitro are discussed. An analytical ultracentrifuge and a spectrophotometer were used to study (a) the breaking of individual polynucleotide strands; (b) the local denaturation; (c) the presence of a fraction of molecules resistant to denaturation; and (d) the increase in the buoyant density of irradiated DNA. All the curves show a slight variation of the radiation efficiency in the range 2600-2800 A, and a well defined peak at lambda = 2880 A.