[Fragment]

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

Licence: In copyright

Credit: [Fragment]. Source: Wellcome Collection.

    1/10
    7 S a degenerate helix, such as a straight line or a circle, Notice that a helix accounts rather naturally for the long repeat distance of the structure, as this would correspond to one turn of the helix. This restriction - that the phosphate-sugar groups are spacially uniform - is a great help in model building as it reduces very considerably the number of possibilities that have to be explored. Indeed at first we were unable to build any satisfactory model consistent with our assumptions, but eventually we arrived at a structure which we now belefive to be correct in its broad outlines. This particular model does not contain just one DNA chain, but a pair of +hem, wound round a common axis. These two chains are linked together by their bases. A base on one chain is joined by weak ph-ysical bonds to a base at the same level on the èther chain, and all the bases are paired of in this way right along the structure. This is shown di&grammatically in Fig. . The general appearance of the structure is shown in a symbolic manner in Fig. , in which the two ribbons represent the phosphate sugar chains, and the pairs of bases holding them together are symbolized as horizontal rods. It will be found that t/iis figure looks exactly the same upside down , and to preserve this feature we have built our model so that the actual sequence of atoms in one ph_josphate-suj?ar chain is in the opposite direction to the corresponding sequence in the other. This is shown