300 ERWIN CHARGAFF 



is not yet within our power to describe ; but there exists some evidence that the 

 nucleic acids, hoxh DNA and PNA, are concerned in these operations. It lies 

 within the scope of these remarks to inquire whether there are any outstanding 

 features that arc common to all nucleic acids, for this may be of value for our 

 understanding of the chemical characteristics of such information systems. In 

 dividing the rest of the discussion under the headings of invariabihty, diversity, 

 and regularity, it will be apparent that the first two qualities probably are com- 

 mon to all specific cell constituents, whereas the third is a most imusual feature 

 of the nucleic acids which apparently is not shared by the other plastic cell 

 substances. 



INVARIABILITY OF NUCLEIC ACIDS 



This is a property that is, often by tenet rather than by experience, held to be 

 common to all macromolecular cell components. How limited our knowledge is, 

 I have already emphasized before; but it is, in general, assumed that a given 

 protein or specific polysaccharide or nucleic acid is invariable in structure in a 

 particular species. Although, as I shall mention below, there exists impressive 

 evidence that the nucleic acids of a cell comprise an entire series of different 

 individuals, the constancy, with respect to characteristic base composition, of 

 total deoxypentose nucleic acids appears well established [2] ; a constancy that 

 seems to apply to preparations from all tissues of the same host [7] or from dif- 

 ferent variants of the same microbial species [8]. 



As concerns pentose nucleic acids a decision appeared more difficult for some 

 time owing to the unsatisfactory nature of isolated PNA preparations. When 

 methods were developed permitting the analysis of total PNA without previous 

 isolation, essentially in the form of nucleoprotein, it could, however, be shown 

 that the nucleotide composition of the total pentose nucleic acid of a given cell 

 also is nearly constant [9, 10]. In more recent experiments on the pentose 

 nucleic acids of Azotobacter vinelandii [11] and of E. coli (unpublished) this 

 remarkable invariability of PNA could also be demonstrated, in the latter case 

 even under a variety of conditions, for instance, the presence or absence of 

 simultaneous protein synthesis and with and without the addition of an excess 

 of a single nucleoside. 



DIVERSITY OF NUCLEIC ACIDS 



Long before detailed chemical analysis had become possible the existence of 

 many different proteins and polysaccharides had been recognized, often through 

 their antigenic or other biological properties, characteristic physical quaUties or 

 some outstanding chemical components. The specific enzymic properties of 

 many proteins also were noted very early. Direct biological tests are not often 

 applicable to nucleic acids. The activity of specific deoxypentose nucleic acids 

 in bacterial transformations is well established. The demonstration of the ex- 

 clusive role of nucleic acids in virus growth and enzyme induction is, in my 

 opinion, as yet far from unequivocal. But on the whole there surely exists enough 

 evidence to speak of a diversity of nucleic acids on biological grounds. 



The chemical diversity of DNA, as shown by widely different proportions of 



