DNA isolated from Coli bacteriophage T1 was irradiated in 0.165 ᴍ NaCl with γ-rays. The molecular weight was determined by measurement of the sedimentation coefficient and viscosity. An analysis of the boundary permits the determination of the sedimentation distribution. The distribution of sedimentation coefficients obtained at various DNA concentrations were extrapolated to zero concentration and were transformed into molecular weight distributions. These were used to calculate the weight average molecular weight Mw and the number average molecular weight Mn.The molecular weight of T1-DNA was found to be 32· 106. After irradiation at a concentration of 200 μg/ml, double breaks as well as intermolecular crosslinks could be determined. The number of double breaks showed a rise with dose that can best be described as composed of a linear and a quadratic term. At low doses the crosslinks increase linearly, the rate being approximately half of that for the linear part of the double breaks. After higher doses, where most of the molecules are degraded, apparently no additional crosslinks are produced. No crosslinks were seen in DNA degraded by DNase. The influence of the DNA concentration on the degradation and the formation of crosslinks is discussed.
New automatic instrument for the determination of viscosity-average molecular weight