24 



TEMPORAL ORGANIZATION IN CELLS 



control sequence is as follows: A',- controls the rate of synthesis of information- 

 ally homologous protein; F,- controls the rate of production of the /th meta- 

 bohte; and M,- controls the rate of synthesis of mRNA of the /th species as well 

 as taking part in metabolic reactions. These quantities are therefore regarded 

 as the essential control variables in the respective biochemical reactions. This 

 involves the assumption that other possible rate-limiting factors such as size 

 of nutrient and energy pools, concentrations of aporepressors, etc., in the cell 

 can be treated as parameters of the "motion" of the variables A',-, 7,-, and M,-; 

 i.e. that these quantities remain constant or change very slowly compared with 

 the dynamic motion of the system variables. Modifications of this basic 

 control scheme, for example when several enzymes forming a biosynthetic 

 sequence are controlled by parallel repression, will be considered later. 



M 



Figure 2. 



More complicated situations will also be considered wherein repression 

 occurs between different components as well as within single components, so 

 that we get strong interactions as shown in Fig. 2. Here a metaboHte controlled 

 by Yi interacts by repression with another genetic locus L2, while a reciprocal 

 interaction occurs from L2 to L^. More complex types of interaction involving 

 many such coupled components will also be considered briefly. 



It would perhaps be proper at this point to consider the whole question of 

 what units should be used for measuring macromolecular populations: 

 whether it is possible to treat the amount of mRNA of a particular informa- 

 tional species, for example, as a continuously-varying quantity or whether the 

 size of such a population necessitates a stochastic representation; and what 

 the functional relationship is between macromolecular concentrations and 

 activities. These are important considerations which can affect fundamentally 

 the representational value of a biochemical analysis. However, we will proceed 

 for the moment as if all the variables to be considered, molecular and macro- 

 molecular, can be treated as continuous variables, postponing until Chapter 6 

 an estimate of the size of the variables A^/and 7,-. As to the units used for these 

 variables, we will use population numbers for the different molecular and 

 macromolecular species, so that the units are simply molecules per cell. The 



