LECTURES IN BIOLOGICAL SCIENCES 



exact replica of the old; it may very well be involved in other 

 chemical changes also. The nature of its chemical activity is, 

 indeed, still very little understood. There seems, however, no 

 reason why one should not hope eventually to understand, and 

 finally to influence, its chemical activities, and thus to obtain 

 some control over the order in which the nucleotides are built 

 into sequences, and in this way eventually over the proteins 

 which the DNA specifies. Already it has been possible to alter 

 the nucleotide sequences in a very crude way; for instance, 

 they have been altered by supplying the cell with chemical 

 analogues of the purine and pyrimidine bases. Such substitu- 

 tion of an abnormal for the normal chemical grouping has so 

 far resulted only in a general increase of undirected mutation, 

 but along these lines a more precise, and more directed, con- 

 trol may eventually be attained. 



In recent years several other phenomena have been studied 

 whose relation to gene mutation is not so clear, but which may 

 ultimately prove very illuminating. On the one hand, there 

 are several immunological phenomena in which changes are 

 induced which persist through several generations of cells, 

 though not usually through several generations of individuals 

 (Medawar, 1956). The induction of immunological tolerance 

 by the administration of an antigen to an early embryo is a 

 particularly striking case. Perhaps of a different type is the 

 phenomenon of graft hybridization, investigated particularly by 

 Glouschenko (1957), and the induction of changes which, it has 

 been claimed, follow the injection of certain tissues into ani- 

 mals, as in the experiments of Kushner (1958) and Benoit 

 et al. (1957). These phenomena, if they can be substanti- 

 ated and shown to occur in a wide range of materials, are 

 of the greatest possible interest. However, it cannot be claimed 

 that their mechanism is as yet at all understood. It is not clear 

 whether we are dealing with something comparable to bacterial 

 transformation, or with something comparable to the alterations 

 which virus or virus-like proteins may undergo when transferred 

 from one species to another, or with something allied to the 

 mechanisms of immunological processes, or even conceivably 

 with true metabolically induced mutations. 



However such problems may eventually be resolved, it ap- 



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