. Cell heredity. Cytogenetics. THE CHEMICAL BASIS OF HEREDITY 13 Cytosine To chain r^^r,o' K. To chain FIGURE 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
. Cell heredity. Cytogenetics. THE CHEMICAL BASIS OF HEREDITY 13 Cytosine To chain r^^r,o' K. To chain FIGURE 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 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. Please note that these images are extracted from scanned page images that may have been digitally en
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