260 



Structure of Viruses / 1 4 : 6 



use of a few longer mutations greatly speeded the mapping of more than 

 200 rll-mutants of T4, because recombination does not occur if the 

 unlocated mutation includes a region covered by the located mutation. 

 The mapping of a large number of rll-mutants in two cistrons in the 

 T4 phage has altered the interpretation of the genetics of higher 

 organisms. In particular, it has made untenable the idea that only a 

 few mutations are possible per gene or even per cistron. Further, this 

 mapping of the rll cistrons of T4 phage particles has supported the 

 fundamental role of DNA in inheritance, including the possibility of 

 recombination between almost every DNA monomer (nucleotide pair) 

 along the chain, and the possibility of a mutation involving only a few 



i 11 



iM*— La- 



-M- 



~ 



v///Awm 



t}MM 



-L\r 



43 



-\ Strain I 



^ Strain 2 



■i Strain 3 



Figure 7. Diagrammatic representation of part of the A 

 cistron for rll for T4 phage. The locations of three mutations 

 are shown on homologous pieces of the DNA chain. After 

 S. Benzer, in The Chemical Basis of Heredity, W. C. McElroy, 

 and B. Glass, eds. (Baltimore, Md. : The Johns Hopkins Press, 

 1957). 



such nucleotide pairs. The interpretation in Chapter 10, that the 

 critical volume for chromosome damage is greater than the muton, is 

 based on the determination of the length of the muton through the 

 mapping of rll-mutants of the T4 E. coli bacteriophage. (The reader 

 may recall from Chapter 10 that there is a critical volume in which 

 ionizations must occur in order to produce a mutation. This volume 

 has a diameter of about 70 A. Because the diameter of the critical 

 volume is larger than the muton, one may conclude that ionizations 

 must occur near the DNA chain, but not necessarily in it, to produce 

 mutations.) 



6. Summary 



Viruses and bacteriophages lie between living and nonliving materials 

 in terms of their size, structure, and behavior. Characteristic viruses 

 infect all known living cells, causing the eventual death of the cell. 



