240 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



(h) Biosynthesis of the Purine Ring 



The use of radioactive isotopes has allowed the various carbon and nitro- 

 gen atoms in the purine ring to be identified with regard to their origin. 

 The purine ring cannot be formed in the living organism from 

 pyrimidines and as the diagram shows all the contributing fragments are 

 small units. 



The biosynthesis of the purine ring takes place in the course of the 

 synthesis of the purine nucleotides, (p. 255). 



C N 



/ \ /'\ 

 HaN C 5 3 CH 



II 



/ \3 



HaN NH 



4-aminoimidazole- 5 -carboxamide 



y'.N 



N/ 



C2 



he 



4C 



r^ \^y-\^ 





NH, 



Glycine 



(i) Biosynthesis of Fatty Acids 

 The mechanism has been described together with the "fatty acid cycle" 

 (p. 196)- 



