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Credit: "DNA". Source: Wellcome Collection.
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![CHAPTER 111 DNA F. H. C. Crick M.R.C. Unit, Cavendish Laboratory University of Cambridge Cambridge, England THE STRUCTURE OF DNA The structure of DNA consists of two long polynucleotide chains, wound together around a common axis. A base attached to one chain is joined by hydrogen bonds to the base opposite it on the other chain, but only certain pairs of bases can be united in this way. They are: Adenine with Thymine or Guanine with Cytosine. Along any one chain any sequence of the bases is possible, but if the sequence along one chain is given then the sequence along the other is automatically determined, because of the pairing rule. It is important to notice that the two chains run in opposite directions: if the sequence of atoms along one of them is con sidered to run up, then along the other it runs down. This structure is now supported by a mass of physicochemical evidence, and especially by the careful and detailed work of Wilkins and his colleagues, using x-ray diffraction. It is also compatible with the analytical rule, first pointed out by Chargaff, that in any sample of DNA the molar amount of adenine equals that of thymine, and that of guanine equals that of cytosine ( or analogue ). Both the x-ray and the chemical results have been shown to [30]](https://iiif.wellcomecollection.org/image/b18174693_PP_CRI_I_1_18_0002.jp2/full/800%2C/0/default.jpg)
