174 CELLS, TISSUES, AND ORGANISMS 



tion, and it could be any one of the preparations shown on our time 

 map, or one as yet undiscovered. 



The only merit such a formulation of the control of division can 

 claim is that it takes advantage of the progress we have made in the 

 analysis of cell reproduction. "Reproduction" and "division" become 

 terms for groups of processes and have value only for abstract discus- 

 sions. "Stimuli" and "controls" act not on "cell division" but only on 

 processes that can be studied individually. Obviously such a treatment 

 of the problems is what we have been striving for, and my point is 

 that we have reached a stage where a more analytical treatment is 

 possible and indispensable. 



Physiological reproduction 



Each daughter cell ideally repeats the history of its mother; to- 

 gether, the two of them transform twice as much of the matter of the 

 outer world into the matter of the biological world. In every cell gen- 

 eration the growth potential doubles. 



This may seem obvious, and easy to explain in terms of the repro- 

 ductive events of the cell cycle, which we have already discussed. For 

 example, we might merely echo the litany of what Dr. Crick has called 

 the Central Dogma. If DNA makes RNA and RNA makes protein and 

 the proteins as enzymes govern the rate of biochemical transformation, 

 then the rate of growth might follow the rate of increase in DNA. It 

 does in a larger sense, but not in a way that describes what happens in 

 the life of a single cell. As we have seen, the DNA doubles during the 

 period between divisions, at least in plant and animal cells. But the 

 growth rate at the time it has completed its doubling is not necessarily 

 twice that at the time before it has begun to double. It may remain 

 about the same, as in Mitchison's studies ( 1957 ) showing a linear 

 growth rate in the yeast Schizosaccharomyces pombe, or in Zeuthen's 

 ( 1953 ) study of the increase in respiratory machinery in Tetrahymena, 

 or in Prescott's ( 1960 ) measurements of protein synthesis in Tetra- 

 hymena from division to division. The instantaneous growth rate of the 

 individual cell may even be declining during the interphase period 

 when DNA is increasing, as in Prescott's (1955) studies on Amoeba. It 

 can double between divisions, as in Paramecium (Kimball et al., 1959) 

 but even here it appears not to parallel DNA increase in any intelligible 

 way (Kimball and Barka, 1959). 



In short, we have no reason to think that the doubling of the 

 growth potential, which I call "physiological reproduction," does 

 parallel genetic reproduction if DNA measures genetic reproduction. 

 In those cases in which the growth rate does not increase exponentially 

 (Mitchison, Zeuthen, Prescott), it doubles suddenly around the time 



