THE NUCLEIC ACID CONTENT OF TISSUES AND CELLS 13 



The ratio PNA-P/DNA-P for liver nuclei is, however, 0.2 to 0.4, and it is 

 possible that a large part of the total PNA of reticulo-endothelial tissues 

 lies within the nucleus, since in these cells the cytoplasm is scanty and the 

 PNA-P/DNA-P ratio is very low. 



When the concentration of PNA-P (including nonnucleotide concom- 

 itants) per unit weight of protein nitrogen (PN) is considered, distinct dif- 

 ferences between organs are no longer apparent. For adult rat liver, kidney, 

 brain, and spleen (Table II), the figures available lie entirely within the 

 range 25 to 37 ng. PNA-P/mg. PN, in contrast to the wide scatter of other 

 measurements on these tissues. Values \\ithin the range 20 to 40 /xg. PNA-P/ 

 mg. PN are also found in mouse and human liver and in human kidney, 

 but exceptions to the rule are mouse submaxillary gland and skin, and brain 

 tissue (Table III). Much more information on this aspect of tissue compo- 

 sition is clearly essential. 



The values for the concentration of DNA-P per unit PN are remarkably 

 similar for homologous tissues in different species. In rat, mouse, and human 

 liver the values are close to 8 /xg- DNA-P/mg. PN, and for the brain tissue 

 of tortoise, duck, fowl, rat, guinea pig, cat, dog, and man (carp is the ex- 

 ception), the values [calculated from results of Bieth and MandeP"] lie 

 within the narrow range of 6.3 to 9.1 ng. DNA-P/)ug. PN.This is all the 

 more significant, as the original figures of Bieth and Mandel for the amounts 

 per cell show DNA, PNA, and PN values ranging from 2.2 to 6.8, 4.4 to 

 26.3, and 26.4 to 88.4 mg., respectively. The relative constancy of DNA 

 per unit PN leads to the conclusion that for one particular type of tissue, 

 the amount of protein per cell is directly proportional to the DNA content 

 of the nucleus, and hence to chromosome substance. 



Mirsky and Ris" have suggested that a similar relationship exists between 

 the cell mass and the DNA-P content per nucleus in homologous tissues. 

 The values for the concentration of DNAP/100 mg. fresh tissue (the re- 

 ciprocal of which is an approximation to the cell mass) give some support 

 to this contention, but they are rather more variable than, for example, the 

 concentration of DNA-P per unit PN in liver and kidney (Tables II and III). 



Table IV shows how the amounts of PNA and DNA in tissues are af- 

 fected by the sex of the animal. In Drosophila salivary gland^- and whole 

 larvae^^ and in fowl liver^^ the PNA-P/DNA-P ratio is greater in the cells 

 of the female than of the male; in the salivary gland, the amounts of 

 DNA-P per cell are not greatly different, and there is more PNA-P per cell 

 in the female than in the male tissue. In each animal the total PNA-P 

 per organ or per larva is also larger in the female than in the male. The 

 situation is completely reversed in rat liver, where, with the exception of 



8' A. E. Mirsky and H. Ris, J. Gen. Physiol. 34, 451 (1951). 

 88 I. Leslie, E. C. R. Reeve, and F. W. Robertson, in press. 



