Summary. 



The enzymatic approaches to the problem of DNA replication and the 

 properties of the DNA-synthesizing enzyme purified from Escherichia coli 

 have been sketched. The unifying and basic generalization about the action 

 of this enzyme is that it catalyzes the synthesis of a new DNA chain in re- 

 sponse to directions from a DNA template; these directions are dictated by 

 the hydrogen bonding relationship of adenine to thymine and guanine to 

 cytosine. The experimental basis for this conclusion is derived from the ob- 

 servations of: (i) The double-stranded character of the enzymatically synthe- 

 sized DNA and its origin from a single stranded molecule, (2) the pattern of 

 substitution of analogues for the naturally-occurring bases, (3) the replication 

 of the chemical composition, (4) the rephcation of the nucleotide (nearest 

 neighbor) sequences and the antiparallel direction of the strands, and (5) the 

 requirement for all four deoxynucleoside triphosphates (adenine, thymine, 

 guanine and cytosine) and DNA for DNA synthesis. 



In closing may I repeat what was said at the banquet last night: Any credit 

 for the work cited here is shared by my colleagues in New York, Bethesda, 

 Saint Louis and Stanford, and by the whole international community of 

 chemists, geneticists and physiologists, which is truly responsible for the 

 progress in nucleic acid biochemistry. 



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