464 



D. O. JORDAN 



Fig. 6. Two models of deoxypentose nucleic acid based on nucleotides of the 

 standard configuration (see Fig. 5) (Furberg"). 



Watson and Crick*^ point out that many of the Van der Waals distances in 

 the Pauhng and Corey structure are too small and also that while the X-ray 

 diagrams are for the sodium salt of deoxypentose nucleic acid, the Pauling 

 and Corey structure refers to the free acid. The great importance of the 

 Pauling and Corey contribution probably lies in the suggestion that the 

 structures of nucleic acids were of the helical type. 



In view of these criticisms a new structure was proposed by Watson and 

 Crick^^ which has many novel features and appears to be in harmony with 

 a large amount of different experimental data, and it would appear, apart 

 from alterations of detail, that this structure is correct for the deoxyribo- 

 nucleate ion as obtained from calf thymus. This structure has two helical 

 chains each coiled round the same axis (see Fig. 7). Both chains follow 

 right-handed helices, but the sequences of the atoms in the two chains run 

 in opposite directions. Each chain resembles the Furberg^^ formula model 

 1 (Fig. 6) and the configuration of the nucleotides are approximate to that 

 given in Fig. 5. The pyrimidines and purines are thus on the inside of the 

 helix and the phosphates on the outside, and a nucleotide occurs in the 

 direction of the long axis every 3.4 A. The structure repeats everj'- ten 

 nucleotides or 34 A., the angle between adjacent nucleotides in the same 

 chain being assumed to be 36°. 



The novel feature of the structure is the manner in which the two chains 

 are held together by the purine and pyrimidine bases. The planes of these 

 bases are perpendicular to the fiber axis and are joined together in pairs by 



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



