BIOLOGICAL ORDER 



Induction and Repression 



Generalized induction hypothesis. The gene z+ contains all 

 of the information for the synthesis of a y8-galactosidase. But the 

 bacterium that possesses the gene z+ does not necessarily manufac- 

 ture the enzyme. When the bacterium is i+ (inductive), an extrinsic 

 inducer is, by definition, necessary. When the bacterium is i"~ 

 (constitutive), the enzyme is synthesized whether or not an extrinsic 

 inducer is present. Why is an inducer sometimes needed for the 

 gene to express itself, that is, to make use of its potentiality by manu- 

 facturing the enzyme? The simplest hypothesis is that the consti- 

 tutive bacterium does not need an extrinsic inducer because it 

 manufactures an inducer itself. This hypothesis can be easily tested. 



One takes a z^i" male (Figure 18). It cannot synthesize enzyme 

 because it does not possess the right information. But it possesses 

 the constitutive gene i~. It could produce enzyme in the absence of 

 an inducer if it were z+. 



One takes a z+i+ female that possesses the right information for 

 the synthesis of the enzyme; but, being i+ (inductive), it is unable 

 to manufacture enzyme in the absence of an inducer. Now males 

 and females are mixed. The male injects its chromosome into the 

 female. If the theory is right, if the function of the gene i~ is to 

 manufacture an inducer, then the zygote should synthesize the 

 enzyme in the absence of an extrinsic inducer. It does not. Things 

 are more complicated than was thought. Another hypothesis has 

 to be considered: the gene i+ manufactures a repressor. 



The repressor. The reverse experiment is then performed. The 

 male is z+i+ instead of the female, and the female is z~i~ instead 

 of the male. In the absence of an inducer, the male cannot manu- 

 facture the enzyme because it is inductive. And the constitutive 

 female cannot manufacture the enzyme because it is z~ . Male and 

 female are again mixed in the absence of an inducer. As soon as the 

 z+i+ gene from the male has penetrated in the z"~i~ female, the 

 heterozygote z+i+/z~i~ starts producing /3-galactosidase at full 

 speed. 



Let us then admit that the gene i+ does synthesize a repressor, 

 the gene i~ being unable to do so. In the zygote type I, the female 

 cytoplasm corresponds to the gene i+, that is, to inductivity. A 



[46] 



