... the forces that held the chains together were salt bridges in which cations such as Mg++ held two or more phosphate groups. In 1952 Franklin responded to Watson and Crick's findings by dismissing the helical structure that she had initially proposed. She also dismissed the way in which the phosphate groups were held together. If they were held together with Mg++ ions then tight shells of water molecules would surround them. IN 1952/1953 Linus Pauling a successful chemist based his theories on the main features of the X-ray diagram. He claimed that DNA was made up of three intertwined helical polynucleotide chains. The phosphate groups are closely packed about the axis of the molecule, with purine and pyrimidine groups projecting radially with their planes being perpendicular to the molecular axis. In 1953 Watson responded to Paulings ideas by saying that the phosphate groups in Pauling's model were not ionised, but that ...

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