GENETIC CONTROL 23 



higher organisms might involve other modes of action of the genes. 

 Moreover, there is evidence that the i gene which controls the inducible 

 or constitutive character of j8-galactosidase can be expressed under such 

 conditions that no protein formation can occur (Pardee et al., 1959); it 

 would seem therefore that the / gene does not act by controlling the 

 structure of any protein (Jacob and Monod, 1959). 



B. CHEMICAL NATURE OF THE GENETIC 

 DETERMINANTS OF PROTEIN STRUCTURE 



1 . Genetic Material of Bacteria and Bacteriophages 



Direct evidence that DNA is the genetic material of bacteria was pro- 

 vided by studies on bacterial transformation. Griffith (1928) had observed 

 that material obtained from encapsulated Pneumococci can confer upon 

 non-encapsulated strains the ability to make a capsule com.posed of specific 

 polysaccharide. Avery et al. (1944) found that transforming activity is 

 associated with DNA. The transforming properties of the DNA prepara- 

 tions are not affected by proteolytic enzymes or by ribonuclease, but they 

 are extremely sensitive to deoxy ribonuclease. The transforming properties 

 also disappear under conditions which cause denaturation of DNA 

 (Zamenhof et al. 1953; Zamenhof, 1956; Lerman and Tolmach, 1959; 

 Lacks and Hotchkiss, 1960). 



Many hereditary characters, including the capacity to make well defined 

 enzymes, can be transferred from one bacterial strain to another by highly 

 purified DNA. This DNA was shown to transfer for instance the capacity 

 to oxidize glucose (Ephrussi-Taylor, 1954) or to make mannitolphosphate 

 dehydrogenase (Marmur and Hotchkiss, 1955). In the transformed bacteria, 

 this capacity is thereafter inherited like any other character and DNA 

 extracted from the transformed bacteria is able to cause transformation. 

 DNA thus performs the two functions which are characteristic of the 

 genes : it brings to the bacterium the capacity of making a specific enzyme 

 and it is duplicated and transmitted to the progeny together with the 

 genetic determinants. The genetic characters introduced by the transform- 

 ing DNA are also capable of recombination (Ephrussi-Taylor, 1951, 1954, 

 1955 ; Hotchkiss and Marmur, 1954). 



Treatment of DNA by nitrous acid is known to deaminate guanine, 

 adenine and cytosine ; when transforming DNA is treated in vitro by this 

 agent and used thereafter as transforming agent, many transformed 

 bacteria have acquired mutated characters. Modifications of DNA brought 

 about in vitro by a chemical agent can thus be expressed as mutations when 



c 



