DNA
... how the rays scattered. Franklin found strong evidence of a helix structure but the photographs suggested that the helix could have two, three or even four strands. She had provided Watson and Crick with further information on their trip to London. Three-chained Failure Watson and Crick’s first model was a triple helix with its bases on the outside of the molecule. The sugar-phosphate backbones ran down the centre, with the phosphate groups of each chain bound to one another by magnesium ions. Watson and Crick hoped that the low water content of the molecule would mean that magnesium ions were in abundance. The men found in difficult making the model fit together, but they discovered that it repeated its structure every 2.8 nanometers along its length. This was very encouraging because this information fit with Franklin’s photograph. Watson and Crick, however, needed to check their model much more accurately against the photograph so they invited Maurice Wilkins and his colleagues from King’s College to the Cavendish Laboratory. When they finally arrived Franklin was still very dismissive of Crick’s support for a helical structure. She pointed out that the proposed three-chained molecule could not possibly exist, as any available magnesium ions would be tightly bonded to water molecule. DNA actually contained ten times at many water molecules as Watson had originally thought. Franklin had stated the correct amount of water molecules when they had earlier spoken to one another in London but, Watson had forgotten on the way back to Cambridge. Their revelation of this incorrect model was embarrassing and Professor Bragg instructed Watson and Crick to get on with their own research and leave model building for the group at King’s College. Although they remained very eager to find the DNA structure, their model building efforts were put on hold. Pauling and Model Building At the start of 1953, Pauling announced that he had discovered the structure of DNA. He proposed a three-chained helix with a sugar-phosphate backbone at the centre. Watson and Crick had also attempted putting the sugar-phosphate backbone in the centre of the molecule when they created their model but the x-ray data collected at King's College in London seemed to suggest the backbones should be on the outside of the molecule. Watson began seriously considering these structures, whose base components would be bound together at the centre of the molecule. When he examined Pauling's structure, he was relieved to see that it could not possibly be correct. The phosphate groups in the backbone of the structure were bound to hydrogen atoms, so were electrically neutral. Such a molecule would not behave as an acid at all so this could not be the structure of DNA. Crick and Watson knew that as soon as Pauling's manuscript was published the mistake would be pointed out and Pauling would begin racing to discover the correct molecular structure. If they were to find it first they would need to hurry. B Type DNA Watson went to London with news of Pauling's error, pointing out that Pauling's mistake was somewhat similar to the mistake he and Crick had made with their DNA model the previous year. Watson's concentration on helical arrangements annoyed Franklin, especially when he implied she was misinterpreting her x-ray photographs. Maurice Wilkins was more sympathetic, however, and told Watson about a new form of DNA that Franklin had discovered that summer. He informed Watson that if DNA were surrounded by lots of water this new 'B' form occurred. Wilkins showed Watson an x-ray photograph of this new structure and they both agreed that the new photograph was incredibly strong evidence for a helix structure. King's College made countless attempts to find a three-chained structure, but had difficulties in making the bases fit together between external sugar-phosphate backbones. Watson decided he would try to find a two-chained structure and began drawing a sketch of Wilkin’s x-ray photograph from memory on his train ride back to Cambridge. Like-with-like Structure After having seen the x-ray photograph of the ‘B’ form of DNA, Watson worked out that DNA was a helix which repeated its pattern along its axis every 3.4 nanometers. He presented this evidence to Bragg and asked permission to build models of possible structures. The professor agreed and models began being built. Data obtained by x-ray diffraction and with electron microscopes showed that the bases were stacked on top of one another 0.3 nanometers apart and at right angles to the axis of the helix. The helix was approximately 2 nanometers in diameter. In their first model, Watson and Crick had tried putting the sugar-phosphate backbone of DNA in the centre of the molecule so they would not need to worry about fitting the bases together. Watson now tried the opposite, building a molecule in the shape of a double helix with the backbones on the outside rather than the inside. Watson thought that the hydrogen bonds might hold the bases together between the backbones. To his surprise, it seemed that hydrogen could bond each of the four bases with another base of the same type. Adenine could bond with adenine, thymine with thymine, cytosine with cytosine, and guanine with guanine. ...