26 NUCLEIC ACIDS AS CELL CONSTITUENTS 



The bacterial cell represents a special case. Whether the micro- 

 bial nucleus is the sole repository of DNA in microorganisms 

 cannot yet be decided with certainty. 



1. BIOLOGICAL SIGNIFICANCE OF NUCLEIC ACIDS 



The discussion of the biological significance of a ubiquitous cell 

 constituent is, strictly speaking, superfluous. But there exist a few 

 important instances pointing to a direct involvement of nucleic 

 acids; and some of them will be listed here briefly. 



All virus preparations so far described are, or contain, 

 nucleoproteins (cf. the recent survey of Davidson^). The same 

 seems to be true of intracellular parasites, such as rickettsiae^- '^ 

 or paramecin^. 



Specific DNA preparations are known which are able to in- 

 duce the transformation of bacterial types. This extremely im- 

 portant phenomenon, first discovered in pneumococci®, has later 

 been shown to operate also in E. coli^^ and in Hemophilus 

 injluenzae^^ {cf. also Ref. 12). The possibility that reactions of 

 this kind are of more general biological importance and not 

 limited to the field of bacterial transformations cannot be reject- 

 ed. What appears particularly remarkable is that it is here the 

 free nucleic acid and not a nucleoprotein (as in the case of viruses) 

 that is able to impose its own synthesis on the receptor cell, 

 whereas in general nucleic acids seem to occur in cells only in 

 the form of conjugated nucleoproteins. The mechanisms through 

 which these transformations take place and the chemical features 

 distinguishing these biologically active DNA specimens are com- 

 pletely obscure. There is, however, httle doubt that it is the 

 bacterial DNA itself, or a particular DNA fraction present in the 

 transforming preparations, which is the carrier of activity. Recent 

 work on the agent operative in the transformation of H. influenzae 

 has shown that highly purified DNA preparations from two types 

 are active in extremely low concentrations: 0.0004 //g of DNA 

 per ml in type b; 0.01 /^g in type c^^. 



The investigation of the relative efficiencies of different wave- 



