Structural and chemical architecture of host cells 107 



by protein synthesis. DNA synthesis occurs between 40 and 72 hours. The 

 mechanisms of these effects are not known, althougli it has been observed 

 that syntheses locahzed in the microsomal fraction are also stimulated. Thus, 

 these experiments show the possibihty of exaggerating further one or another 

 facet of metabohsm, even in differentiated systems. 



C. Synchronous Cultures 



In the past few years, many studies have appeared on the synchronization 

 of division in microbial cultures. These efforts have been sufficiently reward- 

 ing to presage the possibility of comparable results with animal ceU cultures. 

 Zeuthen (1953) started with single Tetrahymena cells and studied synchron- 

 ized growth through four division cycles. During growth, respiration increased 

 linearly rather than exponentially. Linear growth and protem synthesis have 

 also been observed in synchronous cultures of amoebae (Prescott, 1955). 



Scherbaum and Zeuthen (1954) synchronized division in mass cultures of 

 the same organism by raising the temperature 5 degrees above the optimum 

 (29°C.) for one-half hour. All division in progress continued to completion, 

 but growth and the initiation of division stopped. On return to the optimum 

 for 8 hours the cells grew very considerably without division. When the 

 temperature was then lowered to 24°C., 85 % of the cells divided after a lag 

 of 90 minutes. Two further cycles followed at less than 2-hour intervals. 



Hotchkiss (1954) applied the technique of shifting temperatures to pneu- 

 mococci. Bacteria grown at 35°C. were cooled to 25°C. for 15 minutes and 

 returned to 37°C., after which most of the bacteria divided. When a similar 

 method is applied to B. megatherium, DNA synthesis is far less inhibited by 

 chilling than is RNA synthesis, thereby favoring nuclear synthesis in the 

 cold and permitting the apparent phasing of the bacteria (Falcone and 

 Szybalski, 1956; Hunter- Szybalska et al., 1956). 



Lark and Maal0e (1954, 1956) have also used temperature shifts between 

 37 and 25°C. and back to produce synchronous division in Salmonella 

 typhimurium. They found that DNA synthesis slows down selectively at the 

 lower temperature and that elevation to 37°C. selectively speeds up DNA 

 synthesis, leading almost immediately to nuclear division. Throughout these 

 events RNA synthesis remains essentially unchanged. However, following 

 the period of DNA synthesis in each cycle, a period of enhanced ENA 

 synthesis was observed. 



Using the thymine-requiring strain of E. coli, strain 15rr~, Earner and 

 Cohen (1956) were able to synchronize the first division by withholding 

 thymine for a carefully controlled period and then resupplying this pyrimi- 

 dine, essential for DNA synthesis. DNA synthesis began promptly and, after 

 shghtly more than a doubling of DNA, synthesis stopped and was followed 

 by bacterial division. Throughout these events, as seen in Fig. 18, RNA 



