BACTERIOPHAGE GENETICS 355 



mutational event may result in chemical inodification of varying 

 lengths of the nucleic acid chain, and raises the question of the 

 ininimum extent of the mutation. A minimum-extent mutation 

 might be called a point mutation. A point mutation might 

 involve change in a single nucleotide pair, or any number of 

 pairs. The longer span mutations might be a summation of 

 point mutations or might involve some other type of chromosomal 

 aberration such as a deletion or inversion of genetic material. 

 The observation that these long span mutations do not give rise 

 to reverse mutations is consistent with the recombination data in 

 suggesting that extensive chemical alteration is involved. Benzer 

 (1957) proposed the term muton to designate "the smallest element 

 that, when altered, can give rise to a mutant form of the or- 

 ganism." He estimated the size of the muton in phage T4 

 somewhat as follows. The total amount of nucleic acid in 

 phages of the T4 group is about 2X10^ nucleotide pairs per 

 infectious particle as determined by chemical analysis. Certain 

 evidence based on isotope tracer studies (Levinthal, 1956) 

 suggests that only 40 per cent of phage T2 DNA may be in- 

 volved in the transmission of genetic information. Benzer 

 (1955) estimated that the total length of the genetically active 

 nuclear material is about 200 map units (1 map unit gives 1 per 

 cent recombinants in a standard cross) including all known mark- 

 ers. The true map length may differ from this somewhat and 

 a later estimate is 800 map units (Benzer, 1957) after correction 

 for negative interference. If 200 maps units are equivalent 

 to 2 X 10^ nucleotide pairs, then 0.01 map unit (the shortest 

 measured distance separating two mutations) is equivalent to 

 10 nucleotide pairs. As successive resolvable mutations occur 

 at intervals of about 0.01 map unit it is evident that the mini- 

 mum length of the muton may be less than 1 nucleotide pairs 

 and may be only one. Some of the long span mutons mentioned 

 above may be as long as several hundred nucleotide pairs. 

 The lengths of the two measured functional units or cistrons are 

 of the order of 4,000 nucleotide pairs each. The error in these 

 estimates is probably less than a factor of 10. 



