A Structure for Deoxyribose Nucleic Acid(2)

沃森,克里克的杰作

The structure is an open one, and its water Figure 1

This figure is purely content is rather high. At lower water contents

diagrammatic (8). The we would expect the bases to tilt so that the

two ribbons symbolize structure could become more compact.

the two phophate-sugar

The novel feature of the structure is the chains, and the

horizonal rods the pairs manner in which the two chains are held

together by the purine and pyrimidine bases. of bases holding the The planes of the bases are perpendicular to chains together. The

vertical line marks the the fibre axis. They are joined together in pairs,

a single base from one chain being fibre axis. hydroden-bonded to a single base from the

other chain, so that the two lie side by side with identical z-coordinates. One of the pair must be a purine and the other a pyrimidine 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 keto 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), and guanine (purine) with cytosine (pyrimidine) (9). 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.

It has been found experimentally3,4 that the ratio of the amounts of adenine to thymine, and the ratio of guanine to cytosine, are always very close to unity for deoxyribose nucleic acid.

It is probably impossible to build this structure with a ribose sugar in place of the

deoxyribose, as the extra oxygen atom would make too close a van der Waals contact.

The previously published X-ray data5,6 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

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