STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 189 



ultraviolet absorptive properties of the natural and synthetic nucleic acids 

 (Warner, 1957). Hydrogen bonding between bases in the polynucleotides 

 restricts resonance in these structures. As shown by Rich and Da vies (1956) 

 and Warner (1957), synthetic polyadenylic acid and polyuridylic acid in 1 : 1 

 ratio spontaneously aggregate to form a stable, double-stranded compound 

 of lowered absorptive properties, which provides an mteresting model for 

 natural, double-stranded helical DNA. 



If the proportions of polyadenylic acid and polyuridylic acid are varied in 

 the presence of 0.01 M MgClg, it was observed that a stable complex forms 

 at a ratio of 1 polyadenylic acid to 2 uridylic acid (Felsenfeld and Rich, 1957). 

 Thus, a triple stranded compound may be formed in which poly U may fill the 

 helical groove of the normal double-stranded helix. It has been suggested 

 that this may be a prototype for the entrance of a single RNA chain into the 

 heUx of a double-stranded DNA, and perhaps even for its formation at such a 

 site. 



If functional within the cell, the polynucleotide phosphorylase may be 

 expected to produce free RNA polymers. As noted earher, with very few 

 exceptions the nucleic acids are associated with protem withm the ceU. Only 

 a single case is known of the accumulation of free RNA polymers; relatively 

 large particles of free RNA have beenfoimd in some plants (Lindner et al., 1956). 



3. DNA Synthesis 



Kornberg (1957a,b) and his collaborators have isolated and purified an 

 enzyme from E. coli which converts a mixture of deoxynucleoside triphos- 

 phates to DNA, splitting off terminal pyrophosphate in the process (Korn- 

 berg et al., 1956). The reaction will not occur with diphosphates and requires 

 the triphosphates of aU four nucleotides found in DNA. It will not occur to 

 a significant extent in the presence of a single nucleoside triphosphate nor 

 will ATP replace deoxyATP. Of great interest is the requirement for a primer 

 of high molecular DNA, It is thought that such primer DNA may 

 be acting as a template in the model proposed by Watson and Crick. 

 Also that the nucleoside triphosphate may add to the sugar end of the 

 polynucleotide with displacement of inorganic pyrophosphate, in a manner 

 analogous to the relation of primer to the synthetic reaction catalyzed by 

 polynucleotide phosphorylase. Unhke the latter reaction however, DNA 

 synthesis is essentially irreversible, although a very slight exchange of 

 p32.p32 ^^]^ triphosphate occurs in the presence of DNA, Mg ++, and enzyme. 

 Synthetic DNA may attain a particle size comparable to natural DNA. 



The requirement for deoxynucleoside triphosphate has been suggested to 

 account for the absence of uracil in DNA. Thus, thymidylic kinase, which 

 converts thymidyhc acid in the presence of ATP to thymidine triphosphate, 

 is inactive with deoxyuridylic acid (Friedkin and Kornberg, 1957). 



