BIOSYNTHESIS OF NUCLEIC ACIDS 373 



TABLE VI 



Metabolic Inhomogeneity of DNA"-* 



Apparent retentions over a 23-day period of C'"* (from formate) in the nucleic acids 

 of individual organs.*' 



PNA DNA-l" DNA-2' 



G T AG 



1. Small Intestine 1.9 2.7 2.4 2.1 4.1 2.7 3.6 4.2 



" A = adenine, G = guanine, T = thymine. 



° The isotope content of each product on the 24th day is expressed as per cent of the corresponding value 

 on the 1st day. The absolute isotope contents of each product on the Istjay are to be found in Table II, 

 columns 9, 10, and 12. 



•^ Two fractions of the DNA which differ in their ease of sedimentability in 0.87% NaCl.^' 



■^ None of this fraction was obtained from normal liver. 



* No initial incorporation was observed. 



dent renewal of carbons 2 or 8 of the ring of the polynucleotide purines by 

 formate, ^^'^^'^^^ or by the a-carbon of glycine^- did not lend support to this 

 argument. The incorporation of phosphate has frequently shown^^'"'^''23.i34 

 small differences in the renewal of the phosphorus of individual nucleotides, 

 but these differences are from nucleic acid fractions which may well repre- 

 sent mixtures of molecular entities of varying compositions and cannot be 

 considered as proof that individual moieties of a given macromolecule are 

 renewed independently. 



Numerous suggestions have been put forward^^'"*-'''-'^^'^^* to explain 

 the differences in the relative utilizations of the two types of precursors. 

 The consideration of an impermeability of the nuclear envelope to adenine 

 except during mitosis^* must, in view of the ready incorporation of adenine 

 into nuclear PNA (Table V, lines 95 and 100), be at least revised to specify 

 an adenine-containing DNA precursor. The suggestions which now seem 

 most readily susceptible to experimental approach are the possibilities of a 

 heterogeneity of DNA, or that of two mechanisms of synthesis. 



The extent of metabolic inhomogeneity of DNA thus far demonstrated (Table VI) 



133 \Y jj. Marsh, Thesis, Western Reserve University, 1951. 



i3« T. Hultin, D. B. Slautterback, and G. Wessel, Expil. Cell Research 2, 696 (1951). 

 1" G. B. Brown, in "Isotopes in Biochemistry" (Wolstenholme, ed.), p. 164. Blakis- 

 ton, Philadelphia, 1951. 



