HAROLD P. RUSCH 



bonucleic acid (DNA). It is now widely accepted that the 

 pneumococcus transforming principle is a specific type of DNA 

 and that the introduction of this material into a receptor strain 

 of bacteria induces one or more new heritable traits, presumably 

 owing to the production of more of the same type of DNA (3). 

 The phenomenon has been extended to other transformations 

 (33). 



3. Partial hereditary transfer {genetic transduction) by viruses. 

 The most recent of the extra-Mendelian mechanisms has 

 startling implications if the phenomenon can be extended to 

 mammalian cells. It has been found that a bacteriophage 

 grown in one strain of bacteria can infect another strain without 

 killing it and, in so doing, can carry with it (or transduce) a new 

 metabolic property into the new host (38). In all experiments 

 thus far, only one property could be transduced at a time, but 

 by successive transductions as many as five new metabolic charac- 

 ters or reactions could be introduced into and retained by a 

 strain that originally lacked all five of them, thus converting it 

 into five strains. This mechanism may be intrinsically similar 

 to the one described under point (2) if the genetic fragments 

 carried by the bacteriophage are considered to be DNA. 



4. Hereditary change by the establishment of lysogeny. Freeman 

 and Morse (20) demonstrated that an avirulent culture of 

 a diphtheria bacillus could be changed to a virulent one from a 

 symbiotic infection with a bacteriophage. In lysogeny the 

 phage per se produces the heritable alteration in contrast to 

 transduction, where the phage merely acts as the carrier of 

 genetic material from one cell to another. 



Carcinogenesis 



With all these possibilities of cellular change, it is clear 

 that studies on the problem of carcinogenesis have ample in- 

 spiration. Any or all of the above mechanisms could be con- 

 cerned in the conversion of normal cells to cancer cells, 

 and cancer investigators are deeply indebted to the biologists 



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