Nearly four decades after biochemist Phoebus Levene first postulated his "tetranucleotide hypothesis" in 1910, most scientists still believed that DNA was made up of equal numbers of the four nucleotide bases in a repeating tetrameric structure, with each subunit containing all four bases.
Then in 1947, John Masson Gulland, together with Dennis Oswald Jordan and their colleagues at University College, Nottingham, perfected a method of extracting DNA from calf thymus glands. Importantly, their protocol avoided the use of acid or alkali, which kept the solution at a constant neutral pH, allowing them to isolate pure, fibrous, nondegraded DNA (labeled 'desoxyribose nucleic acid' in Gulland's handwriting on the vial at right). When they added strong acids or bases to the sample, however, electrometric titrations showed that hydrogen bonds rather than covalent bonds linked the amino and hydroxyl groups of the nucleotide bases.
These results, along with Erwin Chargaff's ...
