BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 455 



calf thymus nucleic acid^'^'^^^ and wheat germ DNA."^ Certain of the 

 T phages of E. coli are at present the only known natural sources of an- 

 other cytosine base, 5-hydroxymethylcytosine."* These same viruses 

 contain no ordinary cytosine, '^^ whereas the aminopyrimidine of the host 

 bacteria is exclusively cytosine. 



Dramatic as their discovery may be, it is the very exclusiveness of these 

 new bases which indicates that qualitatively different base composition is 

 not the principal foundation for the finer details of DNA specificity. For 

 there are to be explained not only interspecies differences but the existence 

 of different DNA entities within each single species. Phage T2 , having 

 only the hydroxymethyl form of cytosine, builds up a variety of specific 

 DNA-determined factors (see below), as does also the T7 virus (or Pneu- 

 mococcus or Hemophilus) having only cytosine for an aminopyrimidine. 



Similarly, the reported quantitative differences in the base compositions 

 of the DNA from different species need have no connection with the genetic 

 properties of this material. For it is the unfractionated DNA from each 

 species (with the exceptions given below) in which heretofore these analyti- 

 cal differences have been shown. Since these are mixtures, presumably 

 containing a great many different genetically specific entities, there is no 

 indication whether or not these individual components are alike in their 

 composition. What the analyses do show is that differences in the average 

 base composition do occur between species and, therefore, different arrange- 

 ments of bases must be possible in individual DNA molecules. It seems 

 likely that different arrangements of bases in a nucleic acid chain — with 

 or without a difference in overall composition — provide the means by which 

 different specific molecules are constructed. At a still higher level of or- 

 ganization, there may be specific modes of bonding and folding of chains 

 within the macromolecule which create unique structures. This might occur 

 even when the arrangements of the bases along the chains do not greatly 

 differ. 



The most detailed physical structure proposed for DNA, that of Watson 

 and Crick, 1-° has features which not only approximately satisfy the re- 

 quirements of the X-ray data, '-^'2- but also suggest a basis for unique 

 structures and their specific copying in genetic processes. ^-^ The structure 

 is viewed as made up of double coaxial helices held to each other by hydro- 

 gen bonding between complementary pairs of purine and pyrimidine bases 



"« R. D. Hotchkiss, J. Biol. Chem. 175, 315 (1948). 



1'^ G. R. Wyatt, Biochem. J. 48, 581 (1951). 



"8 G. R. Wyatt and S. S. Cohen, Nature 170, 1072 (1952). 



"9 A. Marshak, Proc. Natl. Acad. Set. U. S. 37, 299 (1951). 



120 J. D. Watson and F. H. C. Crick, Nature 171, 737 (1953). 



12' M. H. F. Wilkins, A. R. Stokes, and H. R. Wilson, Nature 171, 738 (1953). 



'" R. E. Franklin and R. G. Gosling, Nature 171, 740 (1953). 



123 J. D. Watson and F. H. C. Crick, Nature 171, 964 (1953). 



