DIVERSITY OF NUCLEIC ACIDS 105 



host' or from different variants of the same microbial species^. 

 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 per- 

 mitting 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^- 1^. In more recent 

 experiments on the pentose nucleic acids of Azotobacter vine- 

 landii^^ and of E. coli^^ 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 simul- 

 taneous protein synthesis and with and without the addition of 

 an excess of a single nucleoside. 



4. 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 qualities 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 

 estabhshed. The demonstration of the exclusive 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 the constituent nucleotides in different specimens, 

 has, since the time of its first discussion^- 1^, been demonstrated 

 in very many instances^. Of more recent date is the discovery that 

 a total DNA preparation can be fractionated into a whole series 

 of differently composed, but regularly graded, fractions^^. The 



References p. 108 



