The control of cell division 



DIVISION WITHOUT GROWTH 



Until we have more definite evidence along these lines, there is much to be said for 

 considering the control of division in cells where growth is not simultaneously in- 

 volved, that is to say in the early stages of totally cleaving eggs. Even here of course 

 it is possible that there is some protein synthesis going on, but it can hardly compare 

 with what occurs during the growth and division of ordinary cells. 



Mainly because of their convenience as experimental material, sea-urchin eggs 

 especially have been used in the study of division. Much is known about their mech- 

 anisms of mitosis and cleavage, and a certain amount about associated chemical 

 changes. This information is mainly derived from a study of inhibitors. In many 

 cases, of course, the chemical action of the inhibitor is uncertain, or even unknown, 

 and the light thrown on the underlying mechanisms of division is therefore not very 

 great. In some cases, however, the action of the inhibitor has been studied in relation 

 to the cell's general metabolism. A certain amount is known, in consequence, about 

 the relation between respiration, the supply of energy and the division process. This 

 work has been reviewed by Krahl (1950). 



The processes of mitosis and cleavage involve mechanical work, so it is not sur- 

 prising to find that division is very dependent on a supply of energy from respiration. 

 In the case of the sea-urchin egg at least it appears that anaerobic glycolysis by itself 

 cannot supply sufficient energy for division. The eggs will not enter division in oxy- 

 gen tensions below about 04 per cent., nor in the presence of inhibitors of the 

 cytochrome system (e.g. carbon monoxide, cyanide, azide) or inhibitors of the Krebs 

 cycle (e.g. malonate) . In this respect they differ from various other eggs, and certain 

 somatic cells, no doubt because their powers of glycolysis are only slight*. Again, 

 as might be expected, neither sea-urchin eggs, nor any other type of cell, will 

 enter division in the presence of agents such as dinitrophenol, which interfere with 

 phosphorylation. There is some evidence, however, that once division is under way 

 it is not inhibited by the normal respiratory or glycolytic poisons. The implication 

 would seem to be that the energy for division is stored up beforehand. 



In spite of this work on the respiratory requirements of division, very little is 

 known about when the energy-producing mechanisms are actually required for the 

 division process. Attempts have of course been made to find variations in respiration 

 during the division cycle, but even the most refined modern techniques of Zeuthen 

 (1950) and Scholander et al. (1952), have shown only minor fluctuations in oxygen 

 consumption, and in many cases, no fluctuations at all. The conclusion seems 

 inescapable that either the energy for division is required in a practically continuous 

 flow, or it absorbs so small a part of the total energy output of the cell as to be scarcely 

 detectable. 



EXPERIMENTS WITH CARBON MONOXIDE ON DIVISION 

 IN SEA-URCHIN EGGS 



With a view to finding out rather more about how the energy-producing mechan- 

 isms support cell division, experiments have been carried out on synchronously 

 dividing eggs, using carbon monoxide as an inhibitor of respiration (Swann, 1953). 



* About 5 per cent, of the sea-urchin egg's ATP supply is derived from glycolysis (Cleland, 1953). 



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