P-P: (P 



v / — A —-" T — \ } 



. \s / I I n/ 



Y^ 7~~ c EEzzE g — ^ ) 



\i / I I I \s 



— x y\ I I I sT 



Y ) — T ZZZ" A — ( 5 



X — ( ! 



I / 'S 



Y — ( ! 



Fig. 7. Mechanism for Enzymatic DNA Replication. 



that further synthesis is blocked for lack of the missing nucleotide. Current 

 studies suggest to us that this limited reaction represents the repair of the 

 shorter strand of a double helix in which the strands are of unequal length, 

 and that the reaction is governed by the hydrogen bonding of adenine to 

 thymine and of guanine to cytosine. 



When all four triphosphates are present, but when DNA is omitted, no 

 reaction at all takes place. What is the basis for this requirement? Does the 

 DNA function as a primer in the manner of glycogen or does it function as 

 a template in directing the synthesis of exact copies of itself? We have good 

 reason to believe that it is the latter and as the central and restricted theme 

 of this lecture I would like to emphasize that it is the capacity for base pairing 

 by hydrogen bonding between the preexisting DNA and the nucleotides added 

 as substrates that accounts for the requirement for DNA. 



The enzyme we are studying is thus unique in present experience in taking 

 directions from a template — it adds the particular purine or pyrimidine sub- 

 strate which will form a hydrogen-bonded pair with a base on the template 

 (Fig. 7). There are five major lines of evidence that I would like to present 

 to support this thesis. 



s-67 



