The control of cell division 



when the supply of energy was short, but which did not divide, would be in a better 

 position to survive or to withstand further stringency than a cell which continued 

 to divide at the expense of growth. In extremis, for instance, accumulated protein can 

 be used as a source of energy, whereas a division that is past and gone cannot. 



It must be emphasized here that an argument of this sort is a biological one that 

 sidesteps the biochemical issues. The exact mechanism that results in maintenance, 

 for instance, having a prior claim on energy supplies, is not known; but in the present 

 context it is not necessary to know. It may be that there is a common pool, in which 

 different activities compete for energy; or it may be that different activities are 

 supplied by different metabolic pathways. In the first case it would have to be 

 supposed that the various activities have differing affinities for the available energy. 

 In the second case the various pathways would have to be differentially sensitive to 

 inhibition or anaerobiosis. In either case, however, the end result is the same from 

 the point of view of the cellular economy. 



This question of the respective claims of division and growth on the cell's energy 

 supply is capable, up to a point, of being settled experimentally. It is in fact highly 

 desirable that the effect of varying degrees of respiratory inhibition in slowing down 

 both growth and division should be investigated. Using either the diver technique 

 or the interference microscope this should not present undue difficulties. Meanwhile, 

 however, it may be worth examining in a little more detail what the possible results 

 of such studies might be. 



If an actively growing cell is subjected by whatever means to a steadily increasing 

 degree of respiratory inhibition, it would be expected, in the light of what has been 

 said above, that division should first slow up, and then stop. Depending on the extent 

 to which the respective claims of division and growth overlap, this slowing up and 

 ultimate stopping of division should be accompanied by some degree of slowing up 

 of growth. Further inhibition will slow up growth yet more, and finally stop it. By 

 this stage it might be expected that maintenance would begin to be affected, at first 

 reversibly. Further inhibition should affect it more drastically, and in due course, 

 irreversibly. 



This sequence of events is illustrated in Figure 2a which shows the suggested 

 apportioning of the cell's total energy supply between maintenance, division and 

 growth. As the available energy decreases, the upper regions of this diagram should 

 be imagined as being cut off. At 50 per cent, of the normal energy supply for instance, 

 very little energy is left for division, whereas there is still a certain amount available 

 for growth, and maintenance is quite unaffected. 



The interesting aspect of this suggested apportioning of the cell's energy supply 

 is that a degree of anaerobiosis or respiratory inhibition, or partial starvation of 

 energy-yielding foodstuffs, will affect division more severely than it will affect growth. 

 Though the cell will therefore grow more slowly, it will grow larger. 



This is not the only possible scheme of things, though it is perhaps the most likely. 

 An apportioning on the lines of Figure 2b for instance, would give a more extreme 

 effect of the kind described above, since division stops entirely before growth is 

 affected at all. In Figure ic, division and growth are affected equally, and cell size 

 should remain constant under all conditions. In Figure id growth is affected before 

 division, so that the cell should get smaller as the energy supply is reduced. 



J9 1 



