In 1951/1952 Francis Crick an English physicist and James Watson an American biologist began their work to discover the structure of DNA. They set about finding how many polynucleotide chains were in the DNA molecule. Using X-ray data they decided that DNA was made up of three chains twisted about each other. They also thought that 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 each group contained a bound hydrogen atom and so had no overall charge. So Pauling’s nucleic acid was not an acid at all. The hydrogen were part of the hydrogen bonds that held together the three chains, without these the chains would form no structure. Watson also claimed that phosphate groups never contained bound hydrogen atoms and so disproving Pauling’s theories.
Pauling later commented on an idea of a double helix structure rather than a triple helix one. The triple helix structure would be very tight allowing little opportunity for change in positions of the atoms.
In 1953 Watson and Crick published a paper in which they discussed the structure of DNA. Their structure was different to those initial ideas. It consisted of two helical chains each coiled round the same axis. Both chains followed right-handed helices but the sequences of the atoms in the two chains run in opposite directions. The bases are on the inside of the helix and the phosphates are on the outside. The purine and pyrimidine bases hold the chains together. The planes of the bases are perpendicular to the fibre axis. They are joined together in pairs by hydrogen bonds. One of the pair must be a Purine and the other a pyramidine for bonding to occur. The base Adenine (purine) joins with Thymine (pryimidine) and Guanine (purine) with Cytosine (pyrimidine).
