14 : 5/ Structure of Viruses 



259 



Another small unit of length used in locating mutations is the muton. 

 This is defined as the smallest unit of length in which a mutation can 

 occur. Again, because of the large number of mutations, lethal on 

 E. coli K but viable on E. coli B, it is possible to obtain better estimates 

 of the muton from studies of the genetics of T4 phage particles than from 

 any other system. The determination of the length of a mutation is 



r769 L 



] rl64 

 1 r895 

 D r832 



O Qn 



— o _ o 



ro rsi rsi LO 



CO 



ro 



rl3l r973 



rl55 



0.018 

 0.019' 



0.073 

 0.20 



0.35,0.37 



0.48 



0.69 



0.16 



0.18 



0.21 



0.22 



0.048 



0.45 



0.48 



0.81 



,0.18 



0.00 — 



0.052 



0.055 



0.20 



0.26 



0.31 



0.32 



SO 



o 

 r596 H 



0.17 



0.19 



0.73 

 0.88 



0.18. 



0.20 



0.060 



Figure 6. A map of the rl64 region of the A cistron for rll- 

 mutations of T4 E. coliphages. The numbers along the hori- 

 zontal lines give the recombination probabilities. The code 

 rl31, for example, means the 131st rll-mutant isolated for T4. 

 After S. Benzer, in The Chemical Basis of Heredity, W. D. 

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

 Hopkins Press, 1957). 



based on recombination probabilities, just as the recon and the cistron 

 are. If one considers three mutations arranged along the DXA chain 

 as shown in Figure 7, it is clear that 



M = L 13 - L i: 



'23 



Because the various lengths are proportional to recombination proba- 

 bilities, one can determine the length M of mutation 2 in terms of 

 probabilities. In this fashion, it has been shown that a mutation may 

 correspond to different lengths from the single muton which corresponds 

 to a probability of less than 0.05 per cent to almost the entire length of 

 the cistron. In absolute length units, the muton is about 20 A. Because 

 mutations can have various lengths, it is possible for them to overlap, 

 that is, cover the same region of the DNA chain. In this case, no recom- 

 binations which lead to wild-type plaques on E. coli K can occur. The 



