ADAPTATION AND SELECTION 65 



1. Adaptive enzymes, which are formed only when growth 

 takes place in the presence of the specific substrate, i.e. are 

 formed only when required. 



2. Constitutive enzymes, which are formed whether 

 growth occurs in the presence or absence of the substrate. 



The application of quantitative studies to enzymes has now 

 shown that adaptive enzymes are usually formed to a small 

 degree even when growth occurs in the absence of the sub- 

 strate, and that the presence of the substrate during growth 

 results in a marked stimulation of the enzyme formation. 

 One explanation that has been put forward of this difference 

 between the enzymes is that the adaptive enzymes are unstable 

 in the absence of their substrate and consequently lose their 

 activity if growth takes place in the absence of the substrate. 

 It is important to realise that this distinction between the 

 two classes of enzymes is an experimental one, as in normal 

 existence an organism will but rarely meet, after growth has 

 ceased, with substances with which it has not been in contact 

 during growth. The adaptive nature of an enzyme can only 

 be shown by taking an organism after growth has taken place 

 in the absence of the substrate, placing it in contact with the 

 substrate, and comparing its activity with that of an organism 

 grown in the presence of the substrate. 



Adaptation of this nature takes place rapidly, for if an 

 organism can ferment galactose adaptively, or, in other words, 

 if it has an adaptive galactozymase, then a single cultivation in 

 the presence of galactose will be sufficient to evoke the enzyme 

 to its full extent. A single subsequent cultivation in the 

 absence of galactose will result in the loss (or marked decrease 

 in the activity) of the enzyme. The difference from an experi- 

 mental point of view between adaptation and selection of 

 mutants is illustrated in Fig. 1 . 



We can summarise the position regarding the formation of 

 an enzyme in the growing cell as follows : the capacity to form 

 the enzyme depends upon the presence of the corresponding 

 gene ; if the gene is present, then the actual formation of the 



CHEM. A. B. 5 



