BIOSYNTHESIS OF NUCLEIC ACIDS 357 



that knowledge of the sequence of intermediates involved and detailed 

 kinetic studies will eventually clarify some of the relationships. 



In considering the available data on the relative incorporations of various 

 precursors into nucleic acids, it must be kept in mind that they are collected 

 under varying experimental conditions and that conclusions derived from 

 comparisons of such data may not always be vaHd. It must also be kept in 

 mind that the present results all represent averages for the nucleic acid 

 "preparations" studied, and that any of the preparations may be susceptible 

 to subfractionation. Individual components of the nucleic acids are observed 

 to be renewed to different extents, and it is necessary to recognize which 

 moiety of the molecule is to be considered. With multicomponent molecules 

 such as the nucleic acids, the metabolic behavior of the molecules as a whole 

 cannot yet be adequately deciphered, since the differences in incorporations 

 observed may reflect either alternatives in the pathways leading to the 

 immediate precursor or metabolic properties of the final molecules per se. 

 It is also now known that the composition of the diet can influence at least 

 liver PNA phosphorus renewal.^" Such conclusions as now appear evident 

 may require frequent reinterpretation as additional knowledge becomes 

 available. 



1. Relative Incorporations into Various Tissues 



a. Incorporations into DMA 's of Individual Organs 



In their pioneer studies Hevesy and co-workers^^'^^ found that the in- 

 corporation of P^--labeled inorganic phosphate into DNA varied widely in 

 different organs of the rat. It was found (Table II, columns 1-3) that the 

 DNA of intestine and spleen were renewed much more rapidly than were 

 those of several other organs. Even greater renewals were observed in the 

 bone marrow and thymus, and the renewal in the lymphoid tissues was 

 found to decrease with the age of the rats.^^ It was recognized that the 

 greatest renewals were to be found in those organs where maximal new 

 cell production occurs. 



It was shown that different values for relative "renewals" would be ob- 

 tained depending upon whether the results were expressed in terms of ac- 

 tivities relative to a single reference value, or relative to the inorganic 

 phosphate of the respective organs (compare Table II, columns 1 and 2, 



9" H. N. Munro, D. J. Naismith, and T. W. Wirkramanayake, Biochem. J. 54, 198 



(1953). 

 « L. Hahn and G. Hevesy, Nature 145, 549 (1940). 

 '2 L. Ahlstrom, H. von Euler, and G. Hevesy, Arkiv Kemi, Minerot. Geol. A19, No. 9 



(1945). 

 " G. Hevesy and J. Ottesen, Acta Physiol. Scand. 5, 237 (1943). 

 s-* E. Andreasen and J. Ottesen, Acta Physiol. Scand. 10, 258 (1945). 



