64 MOLECULES, VIRUSES, AND BACTERLV 



mutant site will give rise to a family of recombinants, all of which ex- 

 clude the mutant site. 



Examination of Figure 8 thus shows that the closer the point of 

 attack to the position of the mutant site, the greater will be its contri- 

 bution to the wild-type recombinant class. If we take the sum of all of 

 the recombination events capable of yielding wild types, we obtain a 

 frequency distribution which has a maximum in the region of short 

 lengths of sequence but is skewed in the direction of long sequences. 

 In other words, the most probable length involved in giving the wild 

 type is short, but a considerable number of total recombinants do 

 arise from the insertion of long sequences, owing to the contribution 

 of the more distant attack points. 



Let us now examine what happens if the mutated sequence in the 

 recipient cell has an appreciable linear dimension. By appreciable 

 linear dimension we shall mean that several points of attack or return 

 are possible within the mutant sequence, i.e., that the mutant sequence 

 is larger than the minimum unit of recombination, which we can as- 

 sume to be a nucleotide. If a mutant containing such a sequence rather 

 than a point mutation is treated by wild-type DNA, fewer wild-type 

 recombinants will be recovered, for any return falling within the 

 mutant sequence will yield a recombinant chromosome which is still 

 mutant. An extension of the mutant sequence prevents events involving 

 short lengths of donor sequence from contributing to the wild-type 

 category of recombinants. Since this is a large proportion of the total 

 of effective recombination events falling around the region of the mu- 

 tation, the recombination frequency will be appreciably reduced. If 

 the mutated site is increased from one minimum unit of recombination 

 to two, a large class of recombination events will now yield mutant 

 structures instead of wild-type, for they will include one of the altered 

 nucleotides of the mutant site. If the site is increased from two to three 

 units, another increment of mutant recombinants will be produced, but 

 this increment will be smaller than that obtained in passing from one 

 to two. In other words, each additional increase beyond two will con- 

 vert a smaller and smaller proportion of recombinant structures into 

 mutant ones. In summary, recombinations yielding the wild type will 

 be lowered by any increase in the dimensions of the mutational site, but 

 this decrease will not be proportional to the linear dimensions of the 

 mutational site. 



Accordingly, the differences in the ease with which the eight 

 mutants at the amylomaltase locus are transformed back to wild type 

 may be considered to reflect the linear extension of the mutant sites. 

 Their size order, judging from the first column of Table I, would be 

 g<'c</</i<d<i</<e. However, if we take into account the errors 

 of the determinations ( see the legend of Table I ) , it turns out that we 



