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- The iconic image of the double helix--the twisted ladder that carries the codes for earth's huge variety of life forms–goes back to 1953 and the homemade metal model created by the British scientist Francis Crick and his American collaborator, James Watson. Determined to solve the puzzle posed by the research evidence at the time, they obtained new insights by visualizing the structure of the complex molecule through a physical model. This pencil sketch of DNA was made by Crick and forms part of the extensive Crick Archive at the Wellcome Library. It illustrates several structural features of the double helix: it is right-handed, with the two strands running in opposite directions; the nucleotides, the building blocks of the strands, have a part that forms the backbone and a part (the base) that projects into the middle of the helix; and the internally projecting bases in one strand are aligned so that they can pair with a base from the opposite strand. This last feature is essential for DNA to be able to perform its function of passing genetic information from one generation to the next. It is not known whether Crick drew this sketch before or after he and Watson made the famous model, but the drawing demonstrates the role that simple illustrations can play in helping to conceptualize complex problems.
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