THE STRUCTURE OF DNA 



J. D. Watson^ and F. H. C. Crick 



Cavendish Laboratory, Cambridge, England 

 {Contribution to the Discussion of Provirus.) 



It would be superfluous at a Symposium on Viruses to introduce a paper on the 

 structure of DNA with a discussion on its importance to the problem of virus re- 

 production. Instead we shall not only assume that DNA is important, but in ad- 

 dition that it is the carrier of the genetic specificity of the virus (for argument, see 

 Hershey, this volume) and thus must possess in some sense the capacity for exact 

 self -duplication. In this paper we shall describe a structure for DNA which sug- 

 gests a mechanism for its self-duplication and allows us to propose, for the first 

 time, a detailed hypothesis on the atomic level for the self-reproduction of 

 genetic material. 



We first discuss the chemical and physical-chemical data which show that 

 DNA is a long fibrous molecule. Next we explain why crystallographic evidence 

 suggests that the structural unit of DNA consists not of one but of two polynucle- 

 otide chains. We then discuss a stereochemical model which we believe satis- 

 factorily accounts for both the chemical and crystallographic data. In conclusion 

 we suggest some obvious genetical implications of the proposed structure. A pre- 

 liminary account of some of these data has already appeared in Nature (Watson 

 and Crick, 1953a, 1953b). 



I. Evidence for the Fibrous Nature of DNA 



The basic chemical formula of DNA is now well established. As shown in 

 Figure 1 it consists of a very long chain, the backbone of which is made up of 

 alternate sugar and phosphate groups, joined together in regular 3' 5' phosphate 

 di-ester linkages. To each sugar is attached a nitrogenous base, only four dif- 

 ferent kinds of which are commonly found in DNA. Two of these — adenine and 

 guanine — are purines, and the other two — thymine and cytosine — are py- 

 rimidines. A fifth base, 5-methyl cytosine, occurs in smaller amounts in certain 

 organisms, and a sixth, 5-hydroxy-methyl-cytosine, is found instead of cytosine 

 in the T even phages (Wyatt and Cohen, 1952). 



^Aided by a Fellowship from the National Foundation for Infantile Paralysis. 



Reprinted by permission of the authors and the Long Island 



Biological Association from Cold Spring Harbor Symposia on 



Quantitative Biology, 18, 123-131 (1953). 



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