330 



CHAPTER 36 



B 



FIGURE 36-1. Electron t7ncroscope photographs of Escherichia coll. A. Whole 

 cells in which nuclear bodies are revealed as less dense areas. Original magnifica- 

 tion 3000X, present magnification about 12,000X. {Courtesy of E. Kellenberger.) 

 B. Thin section showing nuclear bodies and the fine DNA-containing fibers within 

 them. Original magnification 10,000X. Present magnification about 15,000X 

 {Courtesy of W. H. G. Schreil.) 



may be of significance for a study of mutation. 

 One great advantage in the use of bacteria 

 for mutation studies derives from the ease 

 and speed with which large populations can 

 be obtained. For example, under appro- 

 priate culture conditions, E. co// divides about 

 once each half hour. Under such conditions, 

 one cell will produce a clone containing about 

 10 billion (lO^) individuals after the 30 suc- 

 cessive generations that take place in a period 

 of 15 hours. Starting with a single cell, one 

 can calculate the number of E. coli produced 

 after n generations (or t hours) by the expres- 



sion 2° (or 22t) (Figure 36-2). Of course, 

 space is no problem in working with bacteria 

 since 10^" individuals can be grown readily 

 in liquid broth in an ordinary test tube. 



The advantage of small size becomes a 

 handicap, however, when it comes to detect- 

 ing phenotypic changes in bacteria due to 

 mutation. Mutants that change the mor- 

 phology of bacteria must be detected by ex- 

 amining individuals microscopically. Un- 

 fortunately, the single bacterium shows few 

 clear-cut morphological variations. These 

 involve such traits as size, shape, and the 



