I. REPLICATION OF DNA IN CELL-FREE SYSTEMS 



53 



d-AT or d-GC polymers. These reactions were difficult to discern at first, 

 even with the highly purified E. coli enzyme, until the optimal condi- 

 tions were worked out. It is possible that further attempts to synthesize 

 these products with more purified enzyme preparations from calf thymus 

 under suitable conditions will resolve this diff"erence. 



in 



^ 12 5 

 o 



E 



3. 



E 



Salmon sperm DNA 

 (CC/BR) 



Calf thymus DNA 

 . (Worthington) 



/ 



/ 



Heated 



75 



50 



25 



Calf thymus DNA 

 (Volkin via KSD) 



T2 DNA 

 (Volkin via KSD) 



180 2^0 

 Time at 35°C 



Fig. 17. Comparison of heated and unheated DNA preparations with respect to 

 priming capacity. The question mark indicates the uncertainty concerning the 

 manufacturer's statement that "certain less gentle modifications" are used in the 

 preparative procedure. (From BoUum, 1959a.) 



A second difference between the two enzymes, more difficult to resolve, 

 also involves primer. So far, no chemically synthesized polymer has 

 served as primer for E. coli polymerase, and DNase digests of primer 

 DNA are totally inactive in this capacity. Calf thymus polymerase, on 

 the other hand, can use DNase digests as well as synthetic polynucleo- 



