28o ORIGIN OF STRUCTURES AND FUNCTIONS 



desoxynucleotide, guanine desoxynucleotide, cytosine desoxy- 

 nucleotide and thymine desoxynucleotide. Such tetranucleo- 

 tides would have molecular weights of 1,300 for RNA and 

 1,250 for DNA. In fact, however, the values actually found 

 for the molecular weights of various ribonucleic acids ranged 

 from 10,000 to 300,000"^ and for desoxyribonucleic acids 

 from 500,000 to 1,000,000,^" or, according to later results, 

 up to 8,000,000,"* which suggested that nucleic acids are 

 high polymers of tetranucleotides. Naturally, in such poly- 

 mers the quantitative proportions of the various mono- 

 nucleotides and their spatial relations to one another would 

 remain unchanged. This created a very cramped framework 

 for possible variations in the intramolecular structure of 

 nucleic acids. 



However, in the light of recent evidence, which is mainly 

 due to the work of E. Chargaff and his colleagues,"^ the tetra- 

 nucleotide theory of the structure of nucleic acids has been 

 overthrown. It is now accepted that RNA and DNA consist 

 of long chains, the individual links of which are mono- 

 nucleotides joined together by phosphoric acid residues 

 which combine with the hydroxyl groups of the ribose or 

 desoxyribose in the 3 and 5 positions, that is, to give bonds 

 of the type c3'.opo.C5'. The proportions of purine and pyri- 

 midine nucleotides and, even more important, their sequence 

 and orientation in the polynucleotide chain may vary, and 

 do in fact vary extremely widely, in nucleic acids of different 

 origins. 



The evidence from X-ray crystallographic analyses carried 

 out by J. D. Watson and F. H. C. Crick"' suggests a model 

 for DNA which may be represented diagrammatically as in 

 Fig. 25. 



According to this model, the molecule of desoxyribose 

 nucleic acid is composed of two spiral chains wound regularly 

 round a single common axis. Each chain consists of di- 

 esterified phosphate residues combined with ^-D-desoxyribo- 

 furanoside residues in the 3 and 5 positions. The purine and 

 pyrimidine bases lie within the helix while the phosphoric 

 groups are on the outside. The helical chains which make up 

 the molecule are joined together by the paired interaction of 

 the bases of one chain with those of the other. This pairing 



