THE CHEMICAL BASIS OF HEREDITY 



13 



Cytosine 

 To chain r^^r,o' K 



To chain 



FIGURE 1 .3b. Diagram showing the hydrogen bonding between guanine and cytosine 

 in DNA. This pair is held together by three hydrogen bonds in contrast to the two 

 found in the adenine-thymine pair (from Pauling and Corey, 1956, Arch. Biochem. 

 Biophys., 65:164). 



immunological type give rise only to progeny of the same type; type 

 specificity is thus a hereditary property of the organism. 



In Griffith's experiment the heat-killed virulent cells were of immu- 

 nological Type III, and the nonvirulent strain was derived from Type II, 

 as shown in Figure 1.4. No mice died in the control group, which re- 

 ceived only one strain. These results suggest that some interaction 

 occurred between the two strains by which the properties of virulence 

 and type specificity were transferred from the heat-killed virulent strain 

 of Type III to the living, nonvirulent strain, and furthermore, that a 

 hereditary determinant was transferred, since all the progeny bacteria 

 isolated from the dead mice remained heritably virulent and of Type III. 



We can easily formulate this interpretation now, but it was only after 

 intensive experimentation by several investigators that these results were 

 understood. The work culminated in the classical papers of Avery and 

 collaborators on transformation in pneumococcus. 



Experimentation in Avery's laboratory was focused on: (1) repetition of 

 Griffith's results with an in vitro system; and (2) isolation and chemical 

 identification of the transforming substance responsible for converting 

 avirulent cells into virulent ones, and one immunological type into an- 

 other. These aims were achieved. Overcoming great technical difficul- 

 ties, Avery and co-workers were able to demonstrate in vitro type trans- 

 formation with a number of pairs of pneumococcal strains, although the 



