THE DENATURATION OF DNA

Nucleic acids display a very characteristic ultraviolet absorption spectrum with a maximum near 260 nm and a minimum near 230 nm. It was discovered in 1950 that the spectrum of native DNA is much lower than the spectrum calculated from its constituent nucleotides (nt). The observed absorbance strongly increases (hyperchromic effect), not only following hydrolysis, but also following mild treatments, known to respect the phosphoester bonds between nt (and to have little effect on the molecular weight). These results show that DNA has a secondary structure made of secondary valence interactions between the nitrogen bases, and that this secondary structure collapses (DNA denaturation) following mild treatments (low pH, high temperature or, even simply, low salt concentration).