ISOLATION AND COMPOSITION OF PENTOSE NUCLEIC ACIDS 405 



in Table X. It can be seen that different viruses have completely different 

 nucleotide composition. For instance, cytidylic acid varies from 6.6 in 

 potato virus X to 16.8 in turnip yellow mosaic virus. There does not appear 

 to be a regularity comparable to the constant purine-to-pyrimidine ratio 

 of 1, observed in all DNA preparations. [Compare Chapter 10.] The most 

 striking observation made by Knight and his collaborators is that all 

 sixteen strains of tobacco mosaic virus, are identical in composition.^'" 

 Similarlj", the nucleotide composition of two strains of cucumber virus is 

 identical, but different from that of the closely related tobacco mosaic 

 virus.*' These results present impressive evidence that the PNA of plant 

 viruses is species-specific, and that strains which have developed by muta- 

 tion of the parent strain do not differ from it in nucleotide composition. 

 A similar relation was found in the DNA of bacteria: different species 

 possess nucleic acids of different composition, but different strains of the 

 same species yield specimens of the same composition.^^ It has recently 

 been shown that the DNA preparations are composed of a large number 

 of molecules of different composition.''^ The DNA composition of the DNA 

 preparation isolated from an organ or a group of cells represents the aver- 

 age composition of the individuals present in this material. It is quite 

 possible that the viral PNA is similarly composed of a population of mole- 

 cules of different composition. It would appear then that all strains of the 

 same virus contain these nucleic acids in the same proportions. 



The species-specificity of PNA in plant viruses agrees well with the 

 postulate of a genetic function for this PNA. The plant viruses are able 

 to bring about their own production in susceptible cells. Consequently, 

 they must possess a genetic determinant. The lack of DNA, which is con- 

 sidered to be the carrier of heredity in most organisms, suggests that this 

 function is carried out by the PNA present in the virus. 



6. Conclusions 



The results of the determinations of the nucleotide composition of PNA 

 preparations which have been presented in the preceding sections are too 

 scattered, and even contradictory, to permit any definite statements re- 

 garding regularities in PNA composition to be made. However, a few 

 tentative conclusions can be drawn from the results obtained so far, and 

 these may point out the directions for future work. 



(1) PNA may he species-specific. PNA is clearly species-specific in plant 

 viruses: different viral species possess PNA of characteristically different 

 composition, while different strains of the same species possess PNA of 



''* B. Gandelman, S. Zamenhof, and E. Chargaff, Biochim. et Biophys. Acta 9, 399 



(1952). 

 " E. Chargaff, C. F. Crampton, and R. Lipshitz, Nature 172, 289 (1953). 



