The action of γ -rays on sodium deoxyribonucleate in solution. - II. Degradation

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Abstract
Changes in the molecular weight and shape of herring sperm sodium deoxyribonucleate (DNA) which were effected by [gamma] -irradiation in aqueous solution, without exclusion of oxygen, have been followed by means of the light scattering method. The molecular weights (Ml) were proportional to the first power of the intrinsic viscosities of the irradiated solutions, and this confirmed the basis of previous deductions of the mechanism of degradation (Cox, Overend, Peacocke and Wilson 1955, 1958). This relation agrees with the model of a stiff, highly extended, coiled configuration for DNA in solution, which was also indicated by the proportionality of the radius of gyration of the degraded DNA to the square root of the molecular weight. The probability (p) of occurrence of a fragmentation of the molecule as represented by M L1 , was a linear function of the square of the radiation dosage (R) over the dosage range of the experiments (> 1.6 e V/P atom). From this it was deduced (i) that p [image] R2, which confirmed the need for two independent, single breaks in the two chains of DNA to produce a double, fragmenting break, and (ii) that the molecular weight distribution was the same at all R, including R = O, and this implies that the initial molecular weight distribution of the unirradiated DNA was also random. Heating of the irradiated samples caused a further reduction in ML which was greater the larger the previous radiation dosage, but this reduction was much smaller than would have been expected in the absence of any entanglement between the two disordered chains of the DNA. An attempt is made to estimate the extent to which there is a contribution to the process of degradation from single breaks which are not at opposite positions in the two polynucleotide chains.

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