THE HEREDITARY ORDER: GENETIC INFORMATION 



terium that was originally type II. This information was called the 

 "transforming principle." This principle could not be metaphysical; 

 it had to be an organic molecule. 



The bacterial extract containing the transforming principle was 

 fractionated. Each fraction was tested for its transforming ability. 

 Finally, in 1944, Avery, MacLeod, and McCarthy succeeded in 

 obtaining a pure active substance that was identified as desoxyri- 

 bonucleic acid, or DNA. 



Once a bacterium has been "transformed" by a specific DNA, it 

 gives rise to transformed offspring. The daughter bacteria have 

 inherited the specific information. They duplicate it in turn. From 

 them, a transforming principle can be extracted again. Thus the 

 transforming principle, the specific nucleic acid controlling the 

 synthesis of a specific substance, is multiplied and regularly trans- 

 mitted from cell to cell. But this specific nucleic acid is not directly 

 responsible for the synthesis of the polysaccharide. When it is 

 present, the bacterium produces a specific enzyme that can perform 

 a specific reaction, in this case the synthesis of a specific poly- 

 saccharide. 



This is not a unique case but one particular example of a general 

 phenomenon. Many bacteria have been transformed with DNA 

 coming from a closely related form. In all the cases of transforma- 

 tion studied so far, the introduction of the desoxyribonucleic acid 

 of the donor bacterium into the recipient organism endows it with 

 the capacity to synthesize a given enzyme, that is, endows it with a 

 given physiological potentiality. This conclusion has been con- 

 firmed and extended by the study of viruses. The nucleic acid of 

 viruses can be separated from the protein components. When a pure, 

 naked, specific viral nucleic acid penetrates a cell, specific viral 

 proteins and specific viral infectious particles are produced. The 

 nucleic acid of viruses plays the same role in heredity as the nucleic 

 acid of bacteria: both control the synthesis of specific proteins. 



Structure of the Genetic Material 



Desoxyribonucleic acid. Desoxyribonucleic acid, or DNA, con- 

 tains three types of molecules: (a) purine and pyrimidine bases, 

 (b) a pentose, desoxyribose, and (c) phosphoric acid. 



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