402 RADIATION UIOLOOY 



not appear until doses resulting in JO per cent survival have been given 

 and is more pronounced with doses resulting in less than 8 per cent sur- 

 vival. Keillor (U)o2) and Beckhorn {\9rt2) have shown that the exten- 

 sion ot the lai-'; phase is photorevcrsihle by visible light to about the same 

 degree as the bactericidal effects. 



BACTERIAL GENETICS 



Although very little was known concerning bacterial genetics prior to 

 about 1940, the development ol knowledge in this field has been rapid 

 since that time. Inasmuch as excellent reviews are available (Lederberg, 

 1948, 1951; Braun, 1947; Luria, 1947), no extended discussion is neces- 

 sary here. 



The methods of classical Mendclian genetics are not applicable to 

 genetic analysis since, in general, bacteria reproduce asexually. Instead, 

 the chief tool of the bacterial geneticist must be an analysis of mutation. 

 The case for the genie nature of heredity in bacteria, therefore, must rest 

 largely on a number of analogies to the behavior of genes in higher, sexu- 

 ally reproducing organisms. That the fundamental unit of inheritance in 

 bacteria is the gene is strongly indicated by the repeated observations of 

 the permanence of mutated characteristics, the spontaneous occurrence of 

 these characteristics independent of specific environmental stimuli at defi- 

 nite rates, the indepeiulence of different mutations in the same organism, 

 mutation inducibility by mutagenic agents known to be effective in higher 

 organisms, the reversibility of mutations, and the occurrence of mutations 

 with physiological effects similar to known gene mutations in sexually 

 reproducing organisms. 



Luria and Delbriick (1943) showed that the numbers of mutant cells in 

 parallel cultures of bacteria should follow a clonal rather than a random 

 sampling distribution if they resulted from preadaptive mutation. Such 

 was found to be the case for bacteriophage-resistant mutants, and their 

 so-called "fluctuation" test has since been applied successfully to other 

 types of bacterial mutations. More direct demonstrations of the muta- 

 tional origin of bacterial variants have been given by Xewcombe (1949) 

 and Lederberg and Lederberg (1952). 



In addition, Lederberg's thorough analysis (Lederberg, 1947, 1949; 

 Lederberg et al., 1951) of the phenomenon of recombination in E. coli \\\2 

 (Tatum and Lederberg, 1947) shows the physical basis of inheritance in 

 bacteria to be essentially similar to that in higher organisms. Haas et al. 

 (1948) have shown that sublethal doses of ultraviolet increase the fre- 

 fiuency of sexual recombination in strain K12. Although the actual 

 fusion of cells of two mutant strains to form hetero/ygotes has not been 

 demonstrated, Zelle and Lederberg (1951), by means of single-cell studies 



