WATSON AND CRICK 



bases are perpendicular to the fibre 

 axis. They are joined together in pairs, 

 a single base from one chain being 

 hydrogen-bonded to a single base from 

 the other chain, so that the two lie 

 side by side with identical z-co-ordi- 

 nates. One of the pair must be a purine 

 and the other a pyrimidine for bond- 

 ing 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 to- 

 gether. These pairs are: adenine (pu- 

 rine) with thymine (pyrimidine), and 

 guanine (purine) with cytosine (py- 

 rimidine). 



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 se- 

 quence 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 experimentally 

 (Chargafi"; Wyatt, 1952) that the ratio 

 of the amounts of adenine to thymine, 

 and the ratio of guanine to cytosine, 

 are always very close to unity for de- 

 oxyribose 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 

 data (Astburv% 1947; Wilkins and 

 Randall, 1953) on deoxyribose nucleic 



243 



acid are insufficient for a rigorous test 

 of our structure. So far as we can tell, 

 it is roughly compatible with the ex- 

 perimental data, but it must be re- 

 garded as unproved until it has been 

 checked against more exact results. 

 Some of these are given in the follow- 

 ing communications. We were not 

 aware of the details of the results pre- 

 sented there when we devised our 

 structure, which rests mainly though 

 not entirely on published experimental 

 data and stereochemical arguments. 



It has not escaped our notice that 

 the specific pairing we have postulated 

 immediately suggests a possible copy- 

 ing mechanism for the genetic ma- 

 terial. 



Full details of the structure, includ- 

 ing the conditions assumed in building 

 it, together with a set of co-ordinates 

 for the atoms, will be published else- 

 where. 



We are much indebted to Dr. Jerry 

 Donohue for constant advice and 

 criticism, especially on interatomic 

 distances. We have also been stimu- 

 lated by a knowledge of the general 

 nature of the unpublished experimen- 

 tal results and ideas of Dr. M. H. F. 

 Wilkins, Dr. R. E. Franklin and their 

 co-workers at King's College, London. 

 One of us (J. D. W.) has been aided 

 by a fellowship from the National 

 Foundation for Infantile Paralysis. 



REFERENCES 



Pauling, L. and Corey, R. B. Nature, 171: 



346, 1953; Proc. U. S. Nat. Acad. Sci. 39:84, 



1953. 

 Furberg, S., Acta Chem. Scand. 6:634, 1952. 

 Chargaff, E., for references see Zamenhof, S., 



Brawerman, G., and Chargaff, E., Biochiffi. 



et Biophys. Acta 9:402, 1952. 

 Wyatt, G. R., /. Gen. Physiol. 36:201, 1952. 

 Astbury, W. T., Synip. Soc. Exp. Biol. I, 



Nucleic Acid, 66 (Cambridge Univ. Press, 



1947). 

 WUkins, M. H. F., and Randall, J. T., Bio- 



chim. et Biophys. Acta 10:192, 1953. 



