406 B. MAGASANIK 



identical composition. Yeast PNA and animal PNA differ in composition 

 from one another and from viral PNA. However the variations encountered 

 in the composition of PNA preparations isolated from different animals of 

 the same species are as great as the variations encountered in animals of 

 different species. These individual variations in PNA composition remain 

 unexplained. It is unlikely that they are due to genetic differences, for then 

 greater differences between species would be expected. They do not seem 

 to depend on the metabolic state of the organ from which the PNA is 

 isolated, as preparations isolated from normal and from regenerating rat 

 liver did not differ appreciably in composition. It will be necessary to 

 determine critically the range of variation of PNA composition within 

 members of the same species before fruitful comparisons of PNA prepara- 

 tions from different species can be made. 



(2) PNA is presumably not organ-specific. So far no convincing differ- 

 ences in the composition of PNA preparations isolated from different 

 organs of the same animal have been observed. The high guanylic acid and 

 low pyrimidine nucleotide content of pancreas PNA seems to be due to the 

 extensive degradation by pancreatic ribonuclease during isolation, and not 

 to reflect a difTerence in composition between native PNA from pancreas 

 and from other organs. 



(3) The nuclear and cytoplasmic fractions of cells differ in PNA composi- 

 tion. PNA isolated from whole organs consists almost exclusively of cyto- 

 plasmic PNA, as the nuclei are largely removed during isolation, and as 

 nuclear PNA accounts for only 10 % of the total PNA of the cell. [Compare 

 Chapters 18, 19, and 21.] Cytoplasmic PNA does, therefore, not differ 

 appreciably from "whole organ PNA" in composition. The PNA of rat 

 liver nuclei appears to be richer in uridylic acid and poorer in guanylic 

 acid than cytoplasmic PNA. Nuclei isolated from the livers of different 

 rats show greater variation in composition than the corresponding cyto- 

 plasmic fractions. Nuclear PNA is known to be more active metabolically 

 than cytoplasmic PNA . The correlation of metabolic activity with changes 

 in nucleotide composition might prove to be of interest. 



(4) The composition of animal, yeast, and bacterial PNA shows certain 

 regularities. The nucleotide ratios of viral PNA preparations do not fall 

 into an easily discernible pattern. On the other hand, both yeast PNA and 

 PNA of the cytoplasm of animal cells contain purine and pyrimidine 

 nucleotides in nearly equimolar quantities. In animal PNA, guanylic and 

 cytidylic acids predominate, while in yeast PNA the four nucleotides are 

 present in nearly equimolar concentrations. Recently Elson and Chargaff^^ 

 have pointed out that the regularities in the composition of PNA can be 

 demonstrated in a more striking fashion w^hen whole cells or centrifugally 

 prepared cell fractions are subjected to hydrolysis and analysis ensuring 



