112 CONTROL MECHANISMS IN CELLULAR PROCESSES 



allows some genes to be active, others inactive in different phases 

 of cell lite. This is, in essence, what T. H. Morgan ( 1934) proposed 

 in his discussion of embryology and genetics. 



The most dramatic illustration of differential gene activities can 

 be found in the research by Breuer and Pavan ( 1955 ) and Beerman 

 (1952) on the polytenic chromosomes of Diptera. These authors 

 observed that the Balbiani rings and "puffs" of these chromosomes 

 varied both with the developmental stage of the larva and from 

 organ to organ. The authors interpreted this, and the fact that these 

 structures accumulated RNA, as a sign of special gene activity. 

 This interpretation will become absolutely convincing when it will 

 be possible to link these formation sites with the appearance of a 

 specific protein in the cell, and with the genetic locus responsible 

 for the formation of such a protein. 



On a purely genetic level, the control of gene activity has often 

 been assigned to accessory genes such as the controlling elements of 

 McClintock (1956) and the modulator gene of Brink (1954). The 

 nature of these units is obscure; they do not seem to be integral 

 parts of the chromosomes, since they migrate to other sites on the 

 same or other chromosomes. McClintock's controlhng elements can 

 affect gene action at whatever locus they go to, while Brink's muta- 

 tor loses most of its action when removed from the specific R locus, 

 which it can suppress completely. These units do have genetic con- 

 tinuity and it would be attractive to think of them as regulators of 

 gene activity, but it is not easy to see how they can accomplish this 

 in a regular manner. As Brink (Van Schaik and Brink, 1959) says: 

 "One looks in vain in the present data for evidence that the trans- 

 positions of modulators are related to gene activation and deactiva- 

 tion in ways that are meaningful for development of the organism." 

 Also, in order to explain all possible gene functions, the nucleus 

 should contain nearly as many regulator genes as genes themselves. 

 Jacob et at. ( 1960 ) recently introduced a notion of a group of genes 

 with coordinated expression, called "operon." Pardee et al. (1959) 

 found in the system of ;S-galactosidase induction in E. coli that a 

 closely linked regulator gene determines whether the enzyme will 

 be constitutive or adaptive by synthesizing a specific repressor for 

 the system. This repressor acts on another gene, situated next to 

 the genes determining the structure of the enzymes, which then 

 directs the expression of the enzyme genes. By expanding this find- 

 ing into a theory, Jacob et al (1960) suggested that each gene or 



