108 



S. S. COHEN 



synthesis continued substantially unchanged, as did the increment in mass, 

 measured turbidimetrically. Eepeated temperature shifts with Salmonella 

 have synchronized cultures with respect to turbidimetric (mass) changes, as 

 well as to changes of DNA (Bruce et al., 1955). 



Thymine 

 added 



.0 time 

 o'-o 



30 min" 



,fi ^'^O min 



10 



20 



40 



60 80 



(min) 



100 120 



I 

 I 



Fig. 18. Synchronization of DNA synthesis by addition of thymine to a thymine- 

 starved strain of E. coli (Earner and Cohen, 1956). 



Thus, as in normal mitosis in most higher cells, DNA synthesis in syn- 

 chronized cultures of bacteria immediately preceded division. Furthermore, 

 the synthesis of DNA was apparently unrelated to the bulk of RNA syn- 

 thesis. Synchronized cultures of Salmonella have been used to explore 

 lysogeny (Lark and Maalee, 1956), and similar cultures of E. coli were used 

 to study the effect of different parts of the division cycle on T2 multiphcation 

 (Earner and Cohen, 1956). 



A number of other methods of obtaining synchronous cultures of micro- 

 organisms for purposes of following compositional changes have been 

 described (Ogur et al., 1953; Maruyama and Yanagita, 1956; Maruyama, 

 1956). We may note that still other techniques have been employed in rather 

 special animal systems. Thus, fertilization of ova by sperm permits the 



