original paper version 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 (DNA) This structure has novel features which are of considerable biological A structure for nucleic acid has already been proposed by Pauling (4) and C They kindly made their manuscript available to us in advance of Their model consists of three intertwined chains， with the phosphates near the fibre axis， and the bases on the In our opinion， this structure is unsatisfactory for two reasons:(1) We believe that the material which gives the X-ray diagrams is the salt， not the free 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 (2) Some of the van der Waals distances appear to be too 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 This structure as described is rather ill-defined， and for this reason we shall not comment on 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 each chain consists of phosphate diester groups joining beta-D-deoxyribofuranose residues with 3'，5' The two chains (but not their bases) are related by a dyad perpendicular to the fibre Both chains follow right-handed helices， but owing to the dyad the sequences of the atoms in the two chains run in opposite directions (6) Each chain loosely resembles Furberg's2 model N 1 (7); that is， the bases are on the inside of the helix and the phosphates on the The configuration of the sugar and the atoms near it is close to Furberg's "standard configuration，" the sugar being roughly perpendicular to the attached There is a residue on each every 4 A in the z- 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 the fibre axis is 10 A As the phosphates are on the outside， cations have easy access to 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 The vertical line marks the fibre The structure is an open one， and its water content is rather At lower water contents we would expect the bases to tilt so that the structure could become more The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine The planes of the bases are perpendicular to the fibre 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- One of the pair must be a purine and the other a pyrimidine for bonding to The hydrogen bonds are made as follows: purine position 1 to pyrimidine position 1; purine position 6 to pyrimidine position 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 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 The sequence of bases on a single chain does not appear to be restricted in any 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 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 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 The previously published X-ray data5，6 on deoxyribose nucleic acid are insufficient for a rigorous test of our 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 Some of these are given in the following communications (10) We were not aware of the details of the results presented there when we devised our structure (11)， which rests mainly though not entirely on published experimental data and stereochemical It has not escaped our notice (12) that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic Full details of the structure， including the conditions assumed in building it， together with a set of coordinates for the atoms， will be published elsewhere (13)We are much indebted to D Jerry Donohue for constant advice and criticism， especially on interatomic We have also been stimulated by a knowledge of the general nature of the unpublished experimental results and ideas of D M H F Wilkins， D R E Franklin and their co-workers at King’s College， L One of us (J D W) has been aided by a fellowship from the National Foundation for Infantile P
