The control of cell division 



by 



M. M. SWANN 

 Department of Apology, University of Edinburgh 



introduction: division with growth 



Cells maintain a roughly constant size*, from which it has been argued that division 

 must be dependent in some way on growth. Lewis (1948), for instance, says that if 

 this were not so, cells might be expected to get smaller and smaller or larger and 

 larger. It must be admitted, however, that in the few cases where serious attempts 

 have been made to measure the size of cells, it seems that they do sometimes get 

 smaller and smaller or larger and larger. In the growth of protozoan cultures for 

 instance, cell size varies within wide limits at different stages. Comparable variations 

 are to be found in the growth of bacteria and yeast in cultures. Even in the case of 

 somatic cells, where the difficulties of measurement are considerably greater, there 

 are scattered references to variations in cell size under different conditions. 



Nevertheless, to a first approximation, cell size does remain constant. It follows 

 that size plotted against time for a given line of cells must give a relationship of the 

 general type shown in Figure la. The precise form of this relationship has in fact never 

 been settled, because of the extreme difficulty of determining the size of a single cell. 

 There is some evidence from measurements made on Protozoa that the main growth 

 in volume, and perhaps therefore in dry weight, occurs soon after fission, giving a 

 curve of cell size more of the type shown in Figure lb (Calkins and Summers, 1941). 

 This may only be an effect of varying degree of hydration, however, and Zeuthen's 

 work (1953) on respiration in Tetrahymena suggests that synthesis may be more or less 

 continuous. 



Whatever the exact form of the growth curve between one division and the next, 

 it is clear that the processes of mitosis and cleavage are normally triggered off when 

 the size of the cell is roughly double what it was after the previous division. The 

 idea that it is the total size of the cell which somehow provides this stimulus is an 

 old one. It was first put forward by Hertwig (1903) in the form of the nucleo- 

 cytoplasmic ratio, an idea that has been variously acclaimed and criticized (Wilson, 

 1925). The importance of the ratio of cell surface to cell volume or cell mass has also 

 been stressed from time to time (Berrill, 1943). In this connexion Rashevsky (1938) 

 has suggested on theoretical grounds that diffusion forces might cause a splitting in 



* The term 'cell size' is widely used to cover cell volume, total cell mass or weight, and cell dry mass or 

 weight. Such a portmanteau definition may be permissible as a convenience, but it must be emphasized that 

 the most satisfactory measure of individual cell growth is probably dry mass or weight. In certain circum- 

 stances it may be desirable also to measure wet weight or degree of hydration. 



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