V. MOLECULAR MECHANISM OF MUTATIONS 211 



and Chase (1952) had shown that phage T2 injects essentially only its 

 DNA into the bactci-iiim it was possible to correlate genetic observations 

 directly with alterations of the bacteriophage DNA. Other viruses have 

 also played a significant role in this development but the most carefully 

 analyzed tobacco mosaic virus has so far resisted all attempts to effect 

 a genetic analysis, i.e., to obtain recombination between two mutants. 

 Another excellent .system for mutagenic studies should be that of 

 transforming principle (i.e., DNA). DNA could be treated by powerful 

 reagents which cannot be used for phages or cells since they also 

 denature protein ; some of these are so specific that only one of the four 

 bases is attacked. So far no genetic region has been examined in sufficient 

 detail to permit a simple analysis of induced mutations both in the for- 

 ward and the reverse direction. 



C. THE GENETIC SYSTEM OF /'// MUTANTS OF T2 OR T4 PHAGES 



One particular phage system, the rll mutants of phage T4, plays an 

 important role in the following discussion and will therefore be described 

 in more detail. This system has been developed by Benzer (1955) and 

 by Chase and Doermann (1958). The phages are assayed by plating 

 them together with about 2 X 10^ sensitive bacteria, Escherichia coli B, 

 (abbreviated B), on a nutrient agar plate. Each phage that infects and 

 develops in a bacterium produces about 200 new phages which are re- 

 leased by lysis of the bacterium. These phages in turn infect more 

 bacteria and thus a chain reaction produces a halo or "plaque" in the 

 turbid bacterial meadow that is formed after several hours of incubation. 

 Phages of "standard type" produce small plaques with fuzzy edges while 

 rll mutant phages form large plaques with sharp edges. If one phage 

 gives rise to both standard and mutant progeny a "mottled plaque" is 

 formed and one thus has a method to detect forward mutations. Reverse 

 mutations can be selectively measured in a different bacterium, E. coli 

 K12 (A) (abbreviated K), on which rll mutants cannot grow while 

 standard type mutants or revertants can. The plaque types of the phages 

 on the two bacterial strains are summarized in the diagram. 



Bacteria 



K 



p, \ r-^ = Standard type Standard Standard 



rJI type 



Chase and Doermann (1958) have used this selective technique to 

 measure the recombination frequencies between several rll mutants and 



