I. REPLICATION OF DNA IN CELL-FREE SYSTEMS 



47 



for deoxythymidylate since the data indicate that these two nucleotides 

 compete with each other during tlie synthesis. Results of a nearest 

 neighbor analysis on a product synthesized with dUTP*- showed that 

 deoxyuridylate was linked to each of the other deoxyribonucleotides. 



Hydrogen Bonding of Adenine to Thymine 



Vi-H-- 



._..0 



---H 



N = C, 



,N 



/ 



W 



OSUGAR 



Hydrogen Bonding of Guanine to Cytosine 



H 





N 



,H-- 



.-N 



\ 

 C — N 



C-H 

 / 



^N^C. 



oSUGAR 



Fig. 16. Base-pairing relationships of normal constituents and analogs as pre- 

 dicted from the Watson and Crick model for DNA. (From Bessman el al, 1958b.) 



An interesting study of the effect of 5-bromouracil substitution on 

 replication of DNA has recently been reported (Trautner et al., 1962). 

 The d-AT polymer was used to test the frequency with which mistakes 

 in base-pairing occur during DNA synthesis by noting the frequency 

 with which dGMP was incorporated into the d-AT product. Highly 

 radioactive dGTP^- (specific activity =1 X 10^ cpm/^mole) provided 

 the basis for an assay that was capable of detecting the incorporation of 



