RNA AND CONTROL OF CELLULAR PROCESSES 117 



the inducer for tlie activating site: when the inducer occupies the 

 site, the gene functions; when the repressor occupies it this function 

 is prevented. Again, it cannot be exchided that tlie repression acts 

 at the enzyme-forming site, independent of RNA synthesis. 



It is easy to visuahze the vakie of induction and repression for the 

 control of cell activitv. The presence of a precursor starts enzyme 

 production and its disappearance stops it. If a product of enzyme 

 action starts to accumulate, repression stops further enzyme pro- 

 duction. The qualitv of the gene itself would determine what con- 

 centration of inducer or repressor is appropriate for activation to 

 occur or stop. 



Secretory Glands. The essential fact of secretion pertinent to this 

 discussion is that cells of secretory organs produce large quantities 

 of proteins and that this production can be activated or lowered. 

 Of all the possible secretory glands, let us consider the pancreas, 

 since the cvtology of pancreatic enzyme production is being exten- 

 sivelv studied and has been summarized in recent reviews (Hirsch, 

 1958; Hirsch, 1960). 



Several enzymes are produced by the pancreas: trypsin (in the 

 form of trypsinogen ) , amylase, lipase, invertase, nuclease and others. 

 The production and liberation of these enzymes is controlled by an 

 intricate nervous control system, but can also be influenced by drugs, 

 like pilocarpin. Let us consider one particular enzyme-producing 

 cell. The cell is merocrinous, i.e., the cell accumulates the secretion 

 product, empties it and resumes its synthesis. The enzyme-produc- 

 ing part of the cell seems to be ergastoplasm, long ago studied by 

 Garnier ( 1899) . The electron microscope has resolved ergastoplasm 

 as a system of cytoplasmic lamellae carrying RNA particles, ribo- 

 somes. The ergastoplasm is the basophilic part of the cytoplasm 

 and basophilia or, in modern words, RNA content, varies with the 

 functional state of the cell. The enzymes accumulate in "zymogen" 

 granules until they are released into the pancreatic ducts. 



When the pancreatic secretion is activated with pilocarpin, the 

 zymogen granules are released and new enzyme synthesis starts. 

 The most important changes occur in the nuclei (Altmann, 1952). 

 The nucleolus releases its substance into the cvtoplasm ( see above ) . 

 Chromosomes appear to take part in the release of the nucleolar 

 substance. Thereafter, the nucleus increases in size, and new syn- 

 thesis of nucleolar substance begins. It can be seen how the sub- 



