[Fragment]

Date:
[c. 1953]
Reference:
PP/CRI/H/1/17
Catalogue details

Licence: In copyright

Credit: [Fragment]. Source: Wellcome Collection.

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    % essential to have thymine paired with it on the other. It is impossible to fit in guanine or cytosine or a second adenine. In the same way guanine must always be paired with cytosine. On the other hand the model places no restriction on the sequence of pairs of bases as one proceeds along the structure Any 31 specific pair can follow any other specific pair. This is because a pair of bases is flat, and as in this model they are stacked one above another like a pile of coins it does not matter which pair goes above which. This specific pairing of the bases is the direct result of the assumption that both phosphate-sugar chains are helical, ïhis implies that the distance apart of two sugar groups at the same level (one belonging to each chain) is always the same, no matter where one is along the chain. It follows that the bases, which are of course linked to the sugars, have always the same amount of space in which to fit, as can be seen from studying Fig. . If it were not for this restriction the bases cound hy^dro^en-bond together in many different ways. It is the regularity of the phosphate-sugar chains, therefore* which is at the root of the specific pairing Evidence for the mode l The experimental evidence in support of a model of this general type is now considerable. Measurements of the density, and water content of the DNA fibres, taken with the evidence showing how the fibres can be extended in length, strongly