《dna双螺旋结构论文》由会员分享，可在线阅读，更多相关《dna双螺旋结构论文（4页珍藏版）》请在金锄头文库上搜索。
1、A Structure for Deoxyribose Nucleic AcidJ. D. Watson and F. H. C. Crick (1)April 25, 1953 (2), Nature (3), 171, 737-738We wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest.A structure for nuc
2、leic acid has already been proposed by Pauling (4) and Corey1. They kindly made their manuscript available to us in advance of publication. Their model consists of three intertwined chains, with the phosphates near the fibre axis, and the bases on the outside. In our opinion, this structure is unsat
3、isfactory for two reasons:(1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each oth
4、er.(2) Some of the van der Waals distances appear to be too small.Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together by hydrogen bonds. This structure as described is rather ill-d
5、efined, and for this reason we shall not comment on it.We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid (5). This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that e
6、ach chain consists of phosphate diester groups joining beta-D-deoxyribofuranose residues with 3,5 linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow right-handed helices, but owing to the dyad the sequences of the atoms in the two
7、 chains run in opposite directions (6) . Each chain loosely resembles Furbergs2 model No. 1 (7); that is, the bases are on the inside of the helix and the phosphates on the outside. The configuration of the sugar and the atoms near it is close to Furbergs standard configuration, the sugar being roug
8、hly perpendicular to the attached base. There is a residue on each every 3.4 A. in the z-direction. We have assumed an angle of 36 between adjacent residues in the same chain, so that the structure repeats after 10 residues on each chain, that is, after 34 A. The distance of a phosphorus atom from t
9、he fibre axis is 10 A. As the phosphates are on the outside, cations have easy access to them.Figure 1This figure is purely diagrammatic (8). The two ribbons symbolize the two phophate-sugar chains, and the horizonal rods the pairs of bases holding the chains together. The vertical line marks the fi
10、bre axis.The structure is an open one, and its water content is rather high. At lower water contents we would expect the bases to tilt so that the structure could become more compact.The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidi
11、ne bases. The planes of the bases are perpendicular to the fibre axis. They are joined together in pairs, a single base from one chain being 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
12、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 th
13、an 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 assum
14、ptions 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
15、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 su
16、gar 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 communications (10). We were not aware of the details of
