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E. coli bacteriophage. The genetic fine structure of the T4 phage has 

 been determined in more detail than has been possible for any other 

 system. 



The T4 bacteriophage has been used for these studies because it 

 undergoes a particular type of mutation, labeled rll, which is easy to 

 analyze for recombinations. The r-type mutations were originally 

 characterized by their rapid lysis of E. coli strain B. Their genetic 

 character is also shown by the types of plaques formed when plated with 

 various strains of E. coli. The r-type plaque, as shown in Figure 3, is 

 larger than the usual wild-type plaque and has sharper edges. Three 

 different types of r-mutants can be distinguished in terms of the plaques 

 formed with different strains of E. coli, as is described in Table II. One 

 may regard rll as a lethal mutation when the phage is grown on E. 

 coli K. 



TABLE II 

 Plaque Forms when Phage Strains are Plated on Various Host Strains 



* The (m) means minute, turbid plaques ; these are only occasionally formed when rll is 

 plated with E. coli K. 



The three types of r-mutants can be considered to have one genetic 

 character difference. In terminology applied to higher organisms, each 

 type of r-mutant of the T4 phage could be considered to have one gene 

 altered. In this terminology, three different genes, I, II, and III, each 

 lead to the same expression of genetic character, rapid lysis and r- 

 plaques, when the phage is grown on E. coli B. The rll-strain is the 

 most useful for studying (mapping) the fine structure of the gene (or 

 genetic character) because these mutations are lethal on E. coli K but 

 can be grown readily on E. coli B. If two rll-mutant strains of T4 

 phage are mixed and grown on E. coli B and then plated on E. coli K, 

 any genetic recombination can be readily observed by the appearance 

 of wild-type plaques. Thus, in a comparatively small number of 

 experiments the frequency of recombination between the two mutants 

 can be determined even if it is as low as one in 10 7 . 



In fruit flies and other higher organisms, the gene may be divided into 

 units called cistrons. As was discussed in Chapter 10, the cis and trans 



