THE PLAN OF CELLULAR REPRODUCTION 175 



of division. Even where it does increase exponentially, we do not sup- 

 pose that the increase would continue indefinitely without the inter- 

 vention of nuclear division, although this is difficult to test. As an hy- 

 pothesis, let us try out the idea that physiological reproduction is as- 

 sociated with the events taking place at the time of division. 



It obviously does not require the physical distribution of the 

 genetic material into two nuclei, for the growth rate of giant cells with 

 polyploid nuclei may be logarithmic, as the collective growth rate of 

 daughter cells is when division proceeds normally (Whitmore et al., 

 1958, and other studies ) . We have to assign physiological reproduction 

 to something less obvious in the events of cell division. 



One of the appealing hypotheses has invoked the ribosomes— the 

 cytoplasmic centers of protein synthesis. There is no good evidence that 

 they are self-replicating, and we are more inclined these days to trace 

 their active RNA to the nucleus, rather than to autonomous self -repli- 

 cation. In any event, we simply do not observe a sudden doubling of 

 cytoplasmic RNA anywhere in the cell cycle ( Mitchison and Walker, 

 1959; Prescott, 1960 ) . We could make the hypothesis that the non-RN A 

 portions of the ribosomes reproduce, doubling the number of potential 

 sites of synthesis. This has not been tested. Or we could imagine that 

 an event accompanying cell division doubles the number of active ribo- 

 somes without doubling the total number. This begs the question, in 

 the light of our further discussion, but it is not in itself unreasonable. 



The alternative is to re-examine the nucleus with the thought that 

 DNA itself may not be an adequate measure of the physiological capa- 

 bilities of the genetic control center. We have already considered the 

 idea that the replication of the DNA is the act of conception of a new 

 chromosome unit, and that completion of its development to the point 

 where it can separate from its parent unit may take some time. We 

 might suppose that the new unit could not function until it had cut the 

 maternal apron strings. If so, the time when the instantaneous growth 

 rate doubles in some kinds of cells may be the time of completion of the 

 new chromosomes ( Figure 5A ) . We will not have been iconoclasts so 

 far as the Central Dogma is concerned; we will merely have intro- 

 duced some conditions to be met before new DNA goes to work. 



One even more specific hypothesis is possible. The nucleoli have 

 been viewed as regular parts of the chromosome complement, associ- 

 ated with specific chromosome regions. (This view has to be expanded 

 somewhat in view of recent evidence that a characteristic nucleolar 

 component, unidentified chemically but recognized by its affinity for 

 silver under certain conditions, is also associated with the chromosome 

 arms— Tandler, 1959; Das and Alfert, 1960. The classical nucleoli, asso- 

 ciated with the "nucleolus organizer regions," are major but not ex- 

 clusive foci of the characteristic chemistry of the nucleolus.) But the 



