Volume 3

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    acquire the type specificity of the organisms from which the extract was obtained. The property of forming the new capsule is transmitted inde finitely to subsequent generations, and, in addition the substance respon sible for the induction of transformation is itself reduplicated in the transformed cells. It is thus apparent that one is dealing with hereditary bacterial modifications, which are predictable and subject to direct ex perimental control. The importance of identifying the transforming principle, the name given to the active component of the cell free system, was obvious, and in introducing their classic results, Avery, Macleod and McCarty (1944) noted that biologists have long attempted by chemical means to induce in higher organisms predictable and specific changes which thereafter could be transmitted in series as hereditary characters. Among microor ganisms the most striking example of inheritable and specific alterations in cell structure and function that can be experimentally induced and are reproducible under well defined and adequately controlled conditions is the transformation of specific types of pneumococci. . . . The present paper is concerned with a more detailed analysis of the phenomenon of transformation of specific types of pneumococcus. The major interest has centered in attempts to isolate the active principle from crude bacterial extracts and to identify if possible its chemical nature or at least to characterize it sufficiently to place it in a general group of known chemical substances. Chemical analysis of the transforming principle suggested that it com prised deoxyribonucleic acid. That it might be protein or ribonucleic acid was excluded by the failure of trypsin, chymotrypsin or ribonuclease to impair its activity. Confirmation that the transforming principle was DNA was provided by the experiments of McCarty and Avery (1946a), in which deoxyribonuclease was purified and it was shown that it irreversibly destroyed the transforming principle. Demonstration of the general nature of this conclusion came from the improved purification of transforming DNA which McCarty and Avery (1946b) used to achieve transformation of pneumococcus types II and VI as well as the type III previously used. The significance of the identification of genetic material with DNA was immediately realized by Avery, who in 1943 described the isolation of the transforming principle in a letter (quoted by Hotchkiss, 1966). When al cohol reaches a concentration of about 9/10 volume there separates out a fibrous substance which on stirring the mixture wraps itself about the glass rod like thread on a spool and the other impurities stay behind as a granular precipitate. The fibrous material is redissolved and the process repeated several times. In short, this substance is highly reactive and on elementary analysis conforms very closely to the theoretical values of