130 CONTROL MECHANISMS IN CELLULAR PROCESSES 



carrier should be expected to move from cell to cell freely, to pene- 

 trate easily into the nucleus, and to have the ability to activate or 

 inactivate one group of genes. 



Such activators may be hormones, known to play a role in many 

 enzymatic reactions of the cell. Others must be related to embry- 

 onic inductors, elusive substances desperately sought by experimen- 

 tal embrvologists. These latter substances may be able to confer 

 to genes a relatively permanent competence to act in the production 

 of RNA molecules, required for the synthesis of certain proteins at 

 specific times and sites in development. All the above factors may 

 act on genes together to determine their state of activity. 



It is possible that this changing activity of genes was actually 

 observed under the microscope. In the polytenic chromosomes of 

 certain Diptera, "puffs" are formed at specific loci at specific times 

 (Beerman, 1952; Breuer and Pavan, 1955). RNA is accumulated in 

 these puffs (Pavan, 1958; Rudkin and Woods, 1959). One can vis- 

 ualize it as if a little red light flashes whenever a gene becomes acti- 

 vated. 



RNA, once inside the cytoplasm, being part of ribosomes, is now 

 active in building cell proteins. The rate of protein production will 

 be a function of the quantity and of the activity of the RNA particles 

 present. The quantity will depend on the rate of RNA production 

 by the genes and on the rate of degradation in the cytoplasm. The 

 activity will depend on some intrinsic properties of RNA and on a 

 variety of factors which govern cellular metabolism. RNA particles 

 are at the mercy of the precursor supply, of energy donors, of pep- 

 tide-bond forming enzymes and a series of activators and inhibitors. 

 Still, RNA alone carries the information of the amino acid sequence, 

 and therefore, the nature of the enzyme it forms. 



This Symposium has shown that control of cellular function can 

 occur at many levels. It would be oversimplifying if the main con- 

 trolling function was assigned to RNA and its production. The 

 quality and the quantity of enzymes and other proteins formed in 

 the cell starts an intricate interplay of many factors, each controlling 

 another, and the concerted action of them all produces what we see 

 as a living cell or organism. 



