VI 



Genes and Mutations 



Vi 



We have already seen that there is much evidence that the genes, 

 which carry the unit hereditary characters from one generation to 

 another, are located in the chromosomes and as the chromosomes are 

 composed of nucleic acid (dna) and proteins, it was believed that dna 

 must be at least an important constituent of the gene. The first con- 

 crete evidence in support of this view was the discovery of 'transform- 

 ing principles' which consist entirely of dna. The initial observation 

 which led to the discovery of these factors was made by Griffith. The 

 bacterium, Pneumococcus, occurs in two forms, one of which is 

 covered by a capsule of starchy material and the other is bare. Grif- 

 fith found that when live 'bare' bacteria were injected into mice to- 

 gether with dead bacteria of the encapsulated type, the former 

 acquired the ability to grow a capsule and this ability, when once 

 acquired, was transmitted to succeeding generations. In other words, 

 the bare variety which had gone through numerous generations with- 

 out any change, acquired the ability, when treated with non-living 

 material, to grow a capsule of its own. Later it was found by Dawson 

 and Sia that it was not necessary to grow the bacteria in an animal. 

 The same transformation could be brought about in the same way 

 in a test tube. 



Alloway showed that the whole of the dead encapsulated bac- 

 terium was not necessary, but extracts from it were effective. Finally, 

 in 1944, three American workers, Avery, McCleod and McCarty, iso- 

 lated the active agent or transforming principle and showed that it 

 had all the properties of a nucleic acid (dna). Similar observations 

 have since been made with a considerable number of other bacteria 

 which exist in two well-defined forms. The transformation from one 

 form to another can be effected by material, which in every case has 

 turned out to be dna. Once transformed the bacteria continue to re- 

 produce in the transformed state, so that their genetic constitution 

 must have been altered. It is concluded from this that the material 



