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Learning To Think Spatially
(t)he previously published X-ray data on deoxyribose nucleic acid are insufficient for a rigorous test of our structure. So far as we can tell, it is roughly compatible with the experimental data, but it must be regarded as unproved until it has been checked against more exact results. Some of these are given in the following communications. We were not aware of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereochemical arguments. (Watson and Crick, 1953, p. 737)
As we know, the structure is indeed compatible with experimental data and it stands as proven. The power of their reasoning, based on experimental data and stereochemical arguments, is remarkable.
Here, the committee emphasizes the explicitly spatial nature of their solution. They developed a three-dimensional structure that is scaled in both distance and angular terms. They specified the “handedness” of the structure (right handed) and showed how it was repeated at a fixed spatial interval. However, it is more than simply a structural description. In passing, they coyly and correctly noted:
It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. (Watson and Crick, 1953, p. 737)
Although their original figure (Figure ES. 1) is described as “purely diagrammatic,” physical models (see Figure ES. 2) demonstrated the power of their biological, chemical, and spatial intuitions.
FIGURE ES.1 Diagram of the DNA structure. The two ribbons symbolize the two phosphate-sugar chains, and the horizontal rods the pairs of bases holding the chains together. The vertical line marks the fiber axis. Reprinted by permission from Nature, Watson and Crick, 1953, p. 737; Macmillan Publishers Ltd.
FIGURE ES.2 James Watson (left) and Francis Crick (right) with their DNA model. (From A. Barrington Brown/Science Photo Library.) Reprinted with permission from Photo Researchers, Inc.