MACRO MOLECULES 



109 



is impossible, which gives the desoxyribonucleic acids a very great stability. 

 Their general structure is the following : 



Base 



i 



\ 



\ 



Base 



\ 



P 



\ \ 



Base 



'%' 



Base 



■■z' 



\ 1 \ 

 p i p 



\l \ 



'5' 



C,/ 



Q' 



The concept of the helical structure of the desoxyribonucleic acids was 

 first suggested by Pauling and Corey (1953), following research by the 

 X-ray diffraction method. A helical structure which accounts more fully 

 for the experimental facts has been proposed by Watson and Crick (1953) 

 (Fig. 14). It consists of two helical chains rolled around the same axis. 

 The two chains are twisted in parallel but the order of the atoms is inverse. 

 The purines and pyrimidines are directed to the interior and the phos- 

 phate to the outside. Along the helix a nucleotide occupies a distance of 



34 A 



i 



3.4 A 



Fig. 14 (Watson and Crick) — Helical structure of desoxyribonucleic acid. 



