190 S. S. COHEN 



The synthesis of the nucleic acids by the two enzymes described above may 

 be compared in Table XII. 



TABLE XII 



Synthesis of Nucleic Acids 



Substance synthesized I!NA DNA 



Substrates One or more ribonucleoside All four deoxyriboside 



diphosphates triphosphates 



Energetics of the reaction Readily reversible Essentially irreversible 



Other requirements Mg++, low molecular poly- Mg++, high molecular 



ribotide DNA 



Product sensitive to R.NAase DNAase 



4. Nucleic Acid Synthesis in Extracts of Animal Tissues 



Although thymidine may be incorporated into DNA by thymus nuclei 

 (Friedkin and Wood, 1956) and by homogenates of regenerating rat liver 

 (BoUum and Potter, 1957), it is not known if these reactions involved the 

 intermediation of the triphosphate mechanism described above. The purine 

 precursor, 4-amino-5-imidazolecarboxamide, was also shown to be incor- 

 porated into the purines of DNA and RNA in a pigeon hver homogenate 

 (Noguchi and Miura, 1956). 



A number of other workers have demonstrated the incorporation of nucleo- 

 tides into RNA in cell-free homogenates of animal tissues. In the incorpora- 

 tion of orotic acid into rat liver homogenates, a 5' nucleotide rather than a 

 3' nucleotide appeared to be a precursor (Herbert et al., 1957). Low levels of 

 ATP were essential and incorporation was proportional to the amount of 

 microsome fraction added. Mitochondria were unnecessary, if ATP were 

 supplied exogenously. CaneUakis (1957a) has observed an incorporation of 

 uridine-5'-phosphate into RNA in a particle-free cytoplasmic fraction of rat 

 liver. 



Several groups have demonstrated a terminal incorporation of ATP into 

 RNA in soluble systems. Canehakis (1957b) has shown that AMP was linked 

 exclusively to a terminal monoesterified cytidyhc acid of RNA, a result 

 obtained earlier by Heidelberger et al. (1956), who demonstrated that such 

 a moiety is liberated as adenosine following alkaline cleavage. The terminal 

 addition to RNA chains of AMP derived from ATP was similarly observed 

 by Zamecnik et al. (1957) and Paterson and LePage (1957). Edmonds and 

 Abrams (1957) have evidence for two types of incorporation of AMP from 

 ATP into RNA. 



