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4. DNA Models - Background
Towards the late 1940s, more and more came to be known. Erwin Chargaff noticed that in any species he studied, the quantity of adenine was always the same as the quantity of thymine while the amount of guanine was the same as the amount of cytosine. This came to be known as “Chargaff’s ratios”. But what did these rations mean? At around the same time, X-ray diffraction data indicated that DNA was coiled in a helical structure. But how many chains were part of the helix? Did the nucleic acids point in toward the center our face out?
James Watson and Francis Crick deduced the structure of DNA in 1953. There were several events that helped them put together the puzzle. First and foremost, the meticulous X-ray diffraction work of Rosalind Franklin and Maurice Wilkins clearly illustrated that the DNA molecule consisted of 2 strands, a double helix, with the nucleic acids on the inside of the molecule. Moreover, the distance between the strands and the pitch of the helix could be precisely measured. With this information, Watson and Crick were able to build a model of the sugar-phosphate backbone of DNA.
The final step of the solution required the use of cardboard models of the 4 nucleic acids. Watson and Crick cut out precise shapes for each nucleic acid. On the hunch that Chargaff’s rule implied a pairing between adenine-thymine and cytosine-guanine, they played with their puzzle pieces to see how they might fit together. They realized that in just the right orientation, adenine-thymine and cytosine-guanine pairs were almost identical in shape, thus providing equally spaced rungs between the 2 backbones of the ladder.
Thus the structure of DNA can be said to be composed of two sugar-phosphate backbones, oriented in opposite directions to one another (notice how the sugars on one side are upside-down compared to the sugars on the other strand). The sugars are then attached to a nucleic acid. The nucleic acids are paired such that adenine is always matched to thymine with 2 hydrogen bonds while guanine is always matched to cytosine with 3 hydrogen bonds. A matching pair of nucleic acids is called a base pair. The assembly of one phosphate, sugar and nucleic acid is called a nucleotide.