Temporal Regulation in Cellular Processes 



J. Woodland Hastings ^ 



There are at least two distinct classes of cellular processes which 

 may be considered to exhibit temporal regulation. The mitotic cycle 

 may be considered as an example of the first of these. Starting arbi- 

 trarily with a daughter cell from a recent division, there occur an 

 orderly and regulated sequence of physical and chemical events 

 leading to the division of this cell into two cells, each being similar 

 to the cell at the time when we started our consideration. 



There are manv studies concerned with the mitotic cvcle ( Ducoff 

 and Ehret, 1959). Hase, Mihara, and Tamiya (1960) for example 

 schematicallv represent the life cycle in ChJorella as being divided 

 into four major phases which follow in sequence: growth, ripening, 

 maturing, and division. There are not very many generalizations 

 which we can make concerning the mechanism of the control in- 

 volved here. The description of the cycle in terms of four phases 

 suggests that certain events in one phase may trigger the onset of 

 the subsequent phase. On the other hand, there is evidence that 

 cell division depends upon the completion of a certain number of 

 essentiallv independent or parallel requisite chemical steps. Cer- 

 tainly the understanding of the regulator\' mechanisms involved 

 must await better knowledge of the chemical events and their se- 

 quence. 



As examples of temporal regulation, life cycles of organisms may 

 be considered to be essentialh^ similar to the life cvcle of the cell. 

 It is quite evident that patliwavs of biosynthesis change at different 



1 Di\ ision of Biochemistry, University of Illinois, Urbana, Illinois. Studies described 

 here have been supported in part by a grant from the National Science Foimdation. 



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