128 INFLUENCE OF TEMPERATURE Ox\ BIOLOGICAL SYSTEMS 



time the old cells were transferred to new media, and this might have had 

 an effect in inducing the synchronization. 



Recently, a number of reports have appeared describing methods 

 whereby the randomness of cell divisions in exponentially growing cultures 

 may be partially eliminated and the cultures thus synchronized. Many of 

 these have involved temperature changes (6, 8-13, 18, 19, 23), and the 

 interest in the present discussion will center on those systems using tem- 

 perature changes or cycling to effect synchronization. Physical (14, 21) 

 and biochemical ( 1 1 methods have also been used to effect some degree of 

 synchronization, and it has been observed that alternating cycles of light 

 and dark (20) have an effect on the synchronization of some algae. Various 

 investigations on the physiological (3, 10, 13), biochemical (1, 4, 12), 

 and cytological (8, 11, 19) aspects of these synchronized systems have been 

 reported. 



The purpose of this paper is to discuss these synchronization systems 

 particularly in connection with possible influences of temperature and 

 sudden temperature changes on the cell cycle. We shall start by briefly 

 describing a schematic model of the cell division cycle. 



CELL DIVISION CYCLE 



Recent views on the life history of the cell have been discussed by 

 Mazia (15). It has been shown for several microorganisms that cell 

 growth in terms of weight, volume or protein content is most rapid im- 

 mediately following division and thereafter slows down and apjjarently 

 completely stops sometime prior to the next subsequent division. The 

 ])eriod during which there is little or no synthesis is evidently a period of 

 reorganization during which the cell prepares to divide. Although there 

 had previously been a tendency to think of division as being initiated 

 when the cell reaches a certain critical size, there are objections to such a 

 view and the evidence for the existence of a 'trigger' leading up to division 

 is emphasized by Mazia (15). Thus, this period of no synthesis might be 

 subdivided into two periods, a pre-division period and a period during 

 which the cells are dividing. Generally, cells which are already dividing 

 cannot be stopped from dividing, whereas if the trigger is not pulled various 

 treatments will prevent cells from entering the division stage. Although 

 characterization of the trigger remains an outstanding problem, this gen- 

 eral description of the cell cycle seems an appropriate one. This description 

 of the cell cycle is based largely on observations on protozoa such as 

 Amoeba, with which Mazia and his coworkers have done most of their 

 work, and Tetrahymena, which Zeuthen and others have investigated. 

 Measurements have been made using single cells as well as synchronized 

 cultures. 



