HEREDITY IN SOMATIC CELLS 



381 



FIGURE 12.12. Formation of 

 /3-galactosidase in E. co/i in 

 response to different concen- 

 trotions of inducer (from Cohn, 

 in: McElroy and Glass (eds.), 

 The Chemical Basis of Develop- 

 ment, Baltimore, The Johns 

 Hopkins Press, p. 458). 



50 100 



Increase in Bacterial Mass (jug x ml~^) 



on, in an autocatalytic fashion. At low levels of inducer, every cell has 

 a small constant probability of forming a molecule of permease; once it 

 does, the process of induction goes exponentially to completion. The 

 process of enzyme induction is under these conditions an all-or-none 

 process resembling mutation, and is transmitted cytoplasmically to the 

 daughter cells as long as the inducer is present in the environment. 

 This is a good example of an autocatalytic mechanism determining a 

 hereditary difference between cells and independent of any change in 

 chromosomal genes or extrachromosomal particles other than the 

 enzymes themselves. 



It seems likely that mechanisms involving reversible expression of 

 phenotype are the best candidates for the origin of the orderly variation 

 in cell type that leads to normal development. It is conceivable that 

 almost all cells respond to a stimulus from the environment through a 

 nonchromosomal mechanism. Although the possibility must be left 

 open, it seems less likely that cells could behave in such a uniform way 

 through chromosomal mechanisms which may rather give rise to path- 

 ological processes, initiated nonuniformly in single cells. It should not 

 be assumed, however, that those types of variation that are random in 

 their origin need give rise to a disorderly result. The necessary order 

 can be achieved by the cellular environment; population change may be 



