A study of bacterial populations with induced nuclear and cellular divisions 



division there is evidence from phage experiments which suggests that this process 

 may be particularly sensitive to temperature changes. We have observed previously 

 that phage synthesis, which to a very large extent means synthesis of DNA, has a 

 temperature coefficient about twice as high as that of the rate of cell division 

 (Bentzon, Maaloe and Rasch, 1952). DNA synthesis, in general, may therefore be 

 selectively slowed down at low temperature; if so, the DNA precursors should accum- 

 ulate when the temperature is lowered and reach a higher concentration in cells grow- 

 ing at 25 C. than in cells growing at 37° C. To account for the observation that 

 nearly all the cells in a culture undergo nuclear division very soon after the tempera- 

 ture has been raised from 25 to 37 C. the following assumptions are made: firstly, 

 that nuclear division is initiated when the precursor concentration reaches a critical 

 level which depends on temperature; and secondly, that cells which recently have 

 undergone nuclear division at 25 G. are left with a precursor concentration so high 

 that a new nuclear division will be initiated if the temperature is raised to 37° C. 

 This hypothesis is tentative, as already stated, but it should be possible to test it by 

 means of biochemical studies using tracers to follow the assimilation from the medium 

 of phosphorus and adenine, for instance. 



(2) We shall now proceed to consider cell division. In this respect, the most 

 striking observation made on our Salmonella typhimurium strain is that there seems to 

 be an interval of more than half a generation time between nuclear and cellular 

 division. This is most apparent in the experiments in which only one temperature 

 change is involved; here nuclear division occurs in nearly all cells right after the 

 increase in temperature, and the 'burst' of cell divisions corresponding to the nuclear 

 division occurs 12 to 15 minutes later. It looks as if nuclear division actually blocks 

 cell division for a considerable time. The experiments in which synchronous cell 

 division was obtained also suggest that the cells do not divide until a long time after 

 nuclear division has taken place; we shall return to this type of experiment later 

 when discussing the process of lysogenization. 



Our observations on cell division in Salmonella typhimurium have been made under 

 rather artificial growth conditions as far as temperature is concerned. However, we 

 assume that the time relation between nuclear and cellular division, which we have 

 observed, also applies to cells growing at a constant temperature. In support of this 

 assumption, we may cite the observation, made earlier, that the overall division rate 

 in a synchronized culture is almost as high as might be expected on the basis of the 

 division rates at 25 and 37 C. and the times spent at the two temperatures. The 

 regimen employed to obtain synchronization, therefore, seems not to impair the 

 growth and division processes appreciably; this, we believe, would not be the case 

 if the temperature shifts interfered seriously with the natural sequence of events 

 during the division cycle. 



As a basis for later discussion, we may point to the striking difference between the 

 division cycle in cells of higher organisms and that observed in Salmonella typhimurium. 

 Most animal and plant cells are uninuclear, which means that, normally, cell division 

 must follow right after nuclear division; in the multinuclear bacterial cells which 

 we have studied, the opposite seems to be normal. 



(3) Finally, we want to consider the process of lysogenization and its possible 

 relation to nuclear division. It was mentioned earlier that the establishment of the 



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