Nucleotides as Structural Units

The bulk of the cellular nucleotides are in the form of the polymers RNA and DNA. In each case the polymerization process is simply a nucleotidyl transfer reaction of nucleoside triphosphates to form a chain of nucleoside monophosphates in 3, 5 -phosphodiester linkage, with consequent release of pyrophosphate. 

A detailed discussion of the nucleic acid polymerases is beyond the scope of this work. However, a few comments will be made concerning the nucleotide substrates. 

The DNA nucleotidyltransferase of Kornberg is the most studied enzyme in this field, and has been used to synthesize biologically active molecules of phage /)X 174 DNA. There is some question, however, concerning its exact function in intact cells. (See reference 16.) 

DNA nucleotidyltransferase appears to have only one binding site for nucleoside triphosphates, for which all four substrates compete. Not all compete equally well, however, as shown by their dissociation constants dGTP, 12 yM dATP, 33 nM dTTP, 81 uM dCTP, 147 yM 16). These values were determined in the absence of DNA primer and template and may be different in a complete system. 

The triphosphate is bound adjacent to the 3 -hydroxyl group of the terminal nucleotide of the DNA primer and oriented so that it can form a base pair with the template. When the correct base pair has been formed, the 3 -hydroxyl of the growing chain makes a nucleophilic attack on the a-phosphorus of the deoxynucleoside triphosphate. 

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