SYNCHRONIZATION OF CELL DIVISION 

 BY CHANGES IN TEMPERATURE 



Victor G. Bruce, Department of Biology, Princeton 

 University, Princeton, New Jersey 



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F BACTERIA, and some other microorganisms, are allowed to grow in liquid 

 culture to saturation, they reach a stage where they are not growing and 

 dividing but are in a resting stage in which they are still viable. They 

 are quite uniform in shape and size, although they are generally smaller 

 than the growing cells in a rapidly dividing culture. When resting cells 

 are introduced into fresh media they immediately start synthesizing proto- 

 plasm but because of their uniformly small size they do not for some time 

 divide and hence there is a lag in cell division. Since the cells start off 

 more or less in the same stage of development, one might expect that they 

 would divide somewhat synchronously. Ordinarily this is not the case and 

 the absence of synchrony arises as a consequence of the large variability 

 in the division times of individual bacteria. This variability has been 

 measured for some bacteria (17) and is sufficiently large that synchronized 

 bacteria may rapidly get out of synchrony due to this effect alone. Using 

 the simplified assumptions that the distribution of division times is of 

 the normal curve of error type, together with the assumption that the 

 cells are all initially synchronized at the stage immediately following 

 division, one can calculate the form of the resulting growth curve. If 

 there is to be any detectible synchronization effect, the maximum slope 

 of the growth curve, which reflects the rate at which the cells are dividing, 

 should be at least two times the slope of the straight line growth curve 

 representing random exponential growth. In order for this to be the case 

 the standard deviation from the mean of the distribution of division 

 times must be less than 20%. The variances observed (17) for the distribu- 

 tion of division times in several types of bacteria were always considerably 

 more than this. Observations by some workers have shown however that 

 in some cases there may be a slight amount of synchrony of cells coming 

 out of the lag phase. Hegarty and Weeks (51 and Houtermans (7) have 

 reported observations of effects in bacteria which may be interpreted in 

 this way, and McClintock (16; under certain conditions not investigated 

 in any detail, it was observed that Neurospora develops synchronously if 

 transferred from old cultures to fresh media) has observed a similar effect 

 in Xeurospora. In these cases a temperature change was also made at the 



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