DARWINISM AND MODERN GENETICS 



(McElroy and Glass, 1957). One such type of process is trans- 

 formation or transduction in bacteria. This is, however, not 

 a very close model of the origin of new hereditary variation. In 

 transformation or transduction, part of the hereditary material 

 of one strain of bacterium is carried into another strain, into 

 whose genetic endowment it becomes incorporated. Thus, if we 

 take into account the donor strain as well as the recipient, noth- 

 ing new arises; the process is comparable rather to hybridiza- 

 tion than to mutation. 



A phenomenon which, may ultimately prove more enlight- 

 ening in relation to mutation is that of induced enzyme syn- 

 thesis. Bacteria, and to a certain extent other cells also, when 

 placed in contact with chemical substances which could be the 

 substrates of a group of enzymes, often develop the capacity to 

 produce the appropriate enzyme in considerable quantities; this 

 property may persist in the progeny for a considerable number 

 of generations. The process is certainly not as yet fully under- 

 stood. It seems rather probable that we are dealing here more 

 with the activation of already present hereditary potentialities 

 than with the origin of quite new potentialities. However, it 

 may be that this is too pessimistic an interpretation, and that 

 we are sometimes confronted with cases of the directed mod- 

 ification of the hereditary materials by the other constituents of 

 the cell with which they are in metabolic relations. 



Even if it is as yet not certain that such metabolically di- 

 rected mutations are involved in enzyme induction, our under- 

 standing of the chemical nature of the hereditary substance has 

 certainly reached a point where we can begin to think seriously 

 about the metabolic activities of the gene as a constituent of 

 the dynamic system of the cell. There can now be little doubt 

 that genes are essentially composed of deoxyribonucleic acid 

 (DNA) and protein. Most of the evidence, particularly that 

 from bacterial transformation, suggests that it is the DNA which 

 carries the specific character of the gene. This compound is 

 built out of four nucleotides which are joined together in long 

 sequences; two such linear threads are then twisted round one 

 another to form the complete DNA molecule. The DNA is 

 certainly in close chemical relation with protein, and it must 

 also be responsible for the synthesis of new DNA which is an 



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