Life: Its Nature and Origin 



39 



Fig. 17. Diagrammatic representation of the Watson-Crick model of DNA. 

 P, phosphate group; S, pentose sugar group; A, adenine; T, thyamine; G, 

 guanine; C, cytosine. Horizontal parallel lines symbolize hydiogen bonding 

 between complementary bases A, T, G, and C. (After Beadle.) 



and guanine), and two are smaller bases called pyrimidines (thy- 

 mine and cytosine) (Fig. 17). In DNA two of these chains occur 

 parallel with each other, with the sugar-phosphate chains on the 

 outside and the bases of opposing sides not only directed toward 

 each other but opposite members of each pair probably also joined 

 with weak hydrogen bonds ( Crick, 1954 ) . Watson and Crick ( 1953 ) 

 calculated the spatial arrangements of the atoms in such a com- 

 pound and further postulated that in these pairings of bases, one 

 space must be small, the opposite large, and that only the two pairs 



