Conformation of U.V.-irradiated DNA

Summary The effects of u.v. on DNA have been found to be manifold. The initial effect characterized by a fall in absorbance, rise in sedimentation coefficient and appearance of kinks in many places of the molecule has been attributed to dimerization of the pyrimidines and formation of single-strand breaks. Together with the occurrence of kinks, the molecule becomes folded up and assumes a ring-like appearance at several places; this is partly due to intramolecular crosslinks. With an increase in dose a third effect, i.e. double-strand break which results in back-bone scission, comes into play and predominates. This effect is manifested in a further decrease in absorbance, a fall in sedimentation constant, and the appearance of shorter molecules in the electron microscope. At still higher doses, an equilibrium state is reached when both absorbance and sedimentation coefficient remain steady. The resulting molecules were so short that the effect of dimer formation with the increase in dose was negligible.