On the denaturation of deoxyribonucleic acid

Abstract

The denaturation of DNA, brought about through the gradual withdrawal of electrolytes by dialysis of conc. solutions in the cold against distilled water for periods up to 216 h, was studied. In most experiments calf thymus DNA was employed; a few experiments were performed with Escherichia coli DNA. The principal criteria of the loss of secondary structure were the hyperchromicity, which amounted to an increase of 40 % over the initial ε(P), and the precipitability with metal salts. The effects of denaturation could be abolished nearly completely or partially, depending upon the duration of dialysis, by the addition of NaCl, as shown by the behavior of the reconstituted products upon subsequent thermal denaturation. The differences between the denaturation of mammalian DNA by heat and by dialysis are underlined by the observation that when maximum hyperchromicity is reached, after a dialysis of 24 h, the addition of salt yields preparations exhibiting normal profiles of thermal denaturation. Though extended dialysis produces additional changes, not all reversible by salt, the “renatured” product, even after a dialysis of 216 h, exhibits, on being heated, a hyperchromic rise of which more than half occurs at the normal transition temperature (87°). The view that the dialyzed preparations, even at their maximum hyperchromicity, do not exist as entirely segregated single strands is borne out by the finding that the fractional precipitation of denatured DNA by a variety of cations invariably yields preparations exhibiting full base-pairing and the unchanged dissymmetry ratios of the initial DNA.