Fifty Years Ago: DNA: the double helix

Abstract

‘The chemical nature of the genes is unknown. They are probably proteins, because the nucleic acids consist of (few) blocks of tetranucleotides (Levene, 1930). This simple structure makes them unlikely candidates as carriers of the large information needed to act as genes.’ This view^strange as it may seem today ^ was shared by most biologists when I was preparing my exam in biology at the medical school in 1946. In a series of classical papers from 1906 on, Levene’s group and others had identi¢ed D-ribose and 2-deoxy-D-ribose as the sugars occurring in RNA and in DNA, respectively. They had also identi¢ed the purines and pyrimidines, localized the phosphodiester link between the sugars, and classi¢ed the linkage with the bases as glycosidic. Levene’s work culminated in the suggestion of a tetranucleotide structure for both RNA and DNA, i.e. that the nucleic acids are composed of an (unknown) number of building blocks, the tetranucleotides, each consisting of four sugars, four phosphates and one each of the four bases. In his words, ‘the tetranucleotide theory is the minimum molecular weight and the nucleic