12 



CELL HEREDITY 



To chain 



hain 



FIGURE 1.3o. Diagram showing the hydrogen bonding between adenine and thymine 

 in DNA (from Pauling and Corey, 1956, Arch. Biochem. Biophys., 65:164). 



DNA, but rather randomly arranged, with local regions held together by 

 hydrogen bonds between the bases. The one regularity which has been 

 found in RNA base ratios, nameiv, that the total of 6-keto-equals the 

 total of 6-amino-containing bases, suggests the existence of some struc- 

 tural pattern which has not been identified as yet. The absence of other 

 regularities in base ratio supports the view of RNA as a single- rather 

 than a double-stranded molecule. 



Bacterial Transformation 



In 1928, in England, Griffith reported a most remarkable finding. In 

 studying pneumonia, he had injected into mice a mixture of two strains 

 of pneumococcus (Diplococcus pneumoniae); one strain was living but 

 not virulent, inducing no infection, and the other strain was dead, heat- 

 killed (although if it had been alive, it would have induced bacteremia). 

 The mice died of bacteremia in this experiment, and from their bodies, 

 living, virulent pneumococci of Type III were isolated. 



Virulence in pneumococcus depends upon the presence of a polysac- 

 charide capsule around the cells; many capsular types have been identi- 

 fied, differing in their detailed chemical composition. Cells are typed 

 by immunological tests, which distinguish between subtle chemical 

 differences in the complex polysaccharide molecules. Strains of each 



