I. REPLICATION OF DNA IN CELL-FREE SYSTEMS 



11 



since the amount of dHMP formed is greater than the amount of 

 tetrahydrofohite j)resent. Dihydrofolic acid is inactive. The folic acid 

 derivative presumably acts only as a one-carbon carrier in this system 

 and has no function as a reducing agent as it does in thymidylate syn- 



H 



N. Ji HC-CH,-N^[ ^C-glut 



^X^^N^ ^ II 



OH N5 ^ n 



NHg/ 



deoxyribose phosphate 



H 





OH 



-N^r^C-glut 



- I ^ ^ II 







NH, ^^i 



'HOH 



N 



I 



deoxyribose phosphate 



\ 



H 

 HgNi^ Y CHg 



^^ N^ H I' 



NHj 

 N^^^I-CHgOH 



OH 



O^N 

 I 

 deoxyribose phosphate 



Fig. 3. Postulated mechanism for the synthesis of o-hydroxymethyldeoxycytidy- 

 late. (From Friedkin, 1959b.) 



thesis. Friedkin (1959b) has presented a hypothetical scheme for the 

 reaction between deoxycytidylate and formyl tetrahydrofolic acid (Fig. 

 3). This scheme differs from the mechanism proposed for thymidylate 

 (Fig. 2) in that no intramolecular shift of hydrogens occurs and hy- 

 drolysis of the intermediate leads directly to the hydroxymethyl deriva- 

 tive and free tetrahydrofolate. Deoxycytidylate hydroxymethylase, as 



