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Credit: "A Structure for DNA". Source: Wellcome Collection.
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![phosphorus atom from the fibre axis is 10 iL The structure is an open one and its water content will therefore he rather high. As the phosphates are on the outside, cations have easy access to them. The novel feature of the structure is the manner in which the two chains are held together hy the purine and pyrimidine hases« The planes of the hases are perpendicular to the fibre axis. They are joined together in pairs, a single base from one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side-by-side with identical z co-ordinates. One of the pair must be a purine and the other a pyrirrddine for bonding to occur# The hydrogen bonds are made as follows: Purine position 1 to pyrimidine position 1 Purine position 6 to pyrimidine position 6 If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms, (that is with the k©to rather than the enol configurations) it is found that only specific pairs of bases can bond together. These pairs are: Adenine (purine) with Thymine (pyrimidine) Guanine (purine) with Gytosine (pyrimidine) In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine. Similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined. ] if fehe yairla-;.-' ía](https://iiif.wellcomecollection.org/image/b1818084x_PP_CRI_H_1_11_2_0002.jp2/full/800%2C/0/default.jpg)
