515 



Pyrimidities 



5-PRPP PR 



HN 



/^^N^ \ orotidylic 



O H COOH pyrophosphorylase 

 Orotic Acid 



COOH 



OH OH 



Orotidine -5 -phosphate 



P— P— P— OCH2 /O 



lOI 



OH OH 



Uridine-5'- 

 triphosphote 



CO, 



HN 



n 



'K^J 



ATP 



orotidylic 

 decarboxylase POCH.. ,0 



OH OH 

 Uridine-5'- phosphate 



HN^l] 



L-glutamine L-glutamate 



AMP 



ATP ADP + P 



NH3 H,0 



cytidylic 

 deaminase 



P— P— P— O— CHo ^O 

 CMP < 



OH OH 



Cytidine-5'- 

 triphosphate 



The biosynthesis of the deoxyribonucleotides may pro- 

 ceed similarly as far as uridine-5'-phosphate. Direct 

 transfer into the deoxyribose series (i.e. removal of the 

 2'-hydroxyl from the ribose moiety) can then occur, or 

 hydrolysis to the pyrimidine base and subsequent reaction 

 with 2-deoxyribose-l -phosphate can take place. 



There has been much interest in the origin of the 

 5-methyl group in thymine (5-methyluracil). The oc- 

 currence of 5-hydroxymethylcytosine in some species sug- 

 gested donation (in that series) by a tetrahydrofolic acid 

 derivative. Isotope experiments indicate that the a-C- 



