16 Essays in Biochemistry- 



Table 1. Base Composition of DNA Fractions from Various Organs of 



the Adult Rat * f 



* Molar ratios with adenine taken as 1.00. 



t J. R. Fresco, P. J. Russell, Jr., and A. Bendich, unpublished results. 



This possibility was explored, in part, by examining the livers of 

 rats which were subjected to partial hepatectomy. The DNAi to 

 DNA 2 ratio remained high 3 to 23 days following the operation, but, 

 thereafter, the ratio dropped rapidly. The regenerated livers 2 and 3 

 months after partial hepatectomy resembled normal liver in that little 

 or no DNAx could be isolated.* The virtual absence of DNA X in the 

 normal livers of adult rats has been confirmed. It was found, further, 

 that the DNA content of rat liver increases as a result of treatment 

 of rats with alloxan. This increase was attributed specifically to the 

 formation of DNAi, whereas the DNA 2 content was unchanged. 10 



The isolation procedure which permits the isolation of DNAi and 

 DNA 2 was originally based upon an arbitrary concentration of salt, 

 0.87% NaCl. Yet, unexpectedly, it does not seem to be so arbitrary 

 after all inasmuch as it reflects the formation of DNAi in the rapidly 

 growing tissue; i.e., the fractionation procedure appears to have a 

 biological basis in fact in this differentiation of DNA. 



Analysis of the soluble DNA fractions for base composition reveals 

 (Table 1) them to be different from the insoluble ones, and, further, 

 that there are significant differences among tissues. f These results 

 reveal a heterogeneity in the composition of DNA from various organs 

 of the rat and in this regard are in sharp contrast to findings 21 from 

 only a few organs of other mammalian species. The basis of this 

 apparent conflict is not clear at present but may be related to the fact 



* Unpublished results, with J. R. Fresco. 



t Unpublished results, with J. R. Fresco and R. J. Russell, Jr. 



