5. Watson and Crick first built a triple-helix model of DNA. In this model, the nitrogenous bases were on the outside of the DNA molecule, and the phosphate groups were on the inside. Why? Other researchers had first proposed that lipids had a triple helix form They misinterpreted the X- ray crystallography data from Rosalind Franklin They thought that the nitrogenous bases would pair up with each other only if they were on the outside of the molecule.