BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 453 



more rare type of mutation gives in a single step a highly streptomycin- 

 resistant mutant. If we have two strains, one highly resistant as the result 

 of multiple cumulative mutations, the other as the result of the single 

 high step, we can prepare two DNA transforming agents from two strains 

 of different genetic makeup but outwardly similar properties. It is ob- 

 served that each DNA preparation unmistakably reflects the history of its 

 donor strain by transmitting the first level of resistance that strain had 

 reached, which in one case is a low step, in the other a high step. Again 

 we see that the extracted DNA represents not merely the attained resis- 

 tance state of the donor bacteria, but actually appears to be a repository 

 for the successive genetic mutational experiences of the donor strain. 



(4) Transforming Agents as Genes. The bacterial DNA preparations, 

 then, have the most important attributes of genes. It will be remembered 

 that bacterial mutations and therefore bacterial genes had to be considered 

 as less clearly defined than the nuclear genes and mutations of higher 

 forms, chiefly because there is no sexual process generally available with 

 which to test the unitary nature of their transmission. In the very process 

 of transformation, however, we have this unitary transmission of heredi- 

 tary determinants from cell to cell, in the form of DNA particles. Since 

 this criterion is thereby satisfied, the bacterial genes and mutations can 

 be taken to be analogous to those of higher forms, and at least some of 

 the bacterial genes have been found to be in the cell DNA. There were 

 already reasons for suspecting DNA to be important in the genetic mechan- 

 isms of the higher forms, and the results obtained with the bacteria indicate 

 that the transforming agents are composed of DNA and act as if they 

 were bacterial gene material separated from the parent cell. 



There remains to be considered the proper role of the transforming 

 agent in the cell from which it came. It must be pointed out again that 

 mutant properties are merely the convenient markers by which the experi- 

 menter can follow a genetic process. Since each new mutation of bacteria 

 appears to be reflected in the DNA components of the cells, there is every 

 reason to suppose that a host of unidentified normal determinants are pres- 

 ent in this same material. If not disturbed in the parent cell, the marked, 

 and presumably also the unmarked, determinants will be repeatedly 

 reduplicated and, in fact, represented in each of the potentially unlimited 

 progeny of that cell. It is reasonable to conclude that the total array of 

 these DNA determinants is the principal apparatus by which the mother 

 cell passes on to its daughter cell its various inherited potentialities. The 

 DNA particles, then, are not only to be considered as the site of certain 

 mutations and source of certain transforming agents, but also as the 

 fundamental cell components through which, in cell division as well as in 

 fertilization, we suppose most or all properties are inherited in the different 

 species. 



