66 



MOLECULES, VIRUSES, AND BACTERIA 



mutant, such as nia or ???,, and a small mutant, such as niy. It can be 

 seen that the dimensions of the mutated site in the donor molecule are 

 without consequence for the yield.of wild-type recombinants, because 

 any point of attack beginning in the mutant sequence of the donor will 

 of necessity include the extreme right portion of the mutant sequence 

 of the donor. Thus wild-type recombinants will be formed only from 

 points of attack to the right of the end of the mutant sequence of the 

 donor. Consequently, irrespective of the size of the mutated site in the 

 donor DNA molecule, the frequency of wild-type recombinants will be 

 determined by the length of segment separating the donor and recipient 

 cells' mutational sites. If, however, it is the recipient cell that contains 

 the extended mutant sequence, then the number of wild-type recom- 



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J I i_ 



^ 



I I I I I 

 V V \i^ \J/ V y 



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Recipient 



B. 



J u 



II.-. 

 V V W V 



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-D Recipient 



Figure 9. Model of transformations involving two mutants of unequal dimen- 

 sions. A: The size of the marker has no effect on the number of wild type 

 recombinants, since any point of attack starting to the left of position 4 will 

 necessarily include the last of the mutated sequence in any segment ending 

 to the right of the mutant site of the recipient cell. B : The size of the marker 

 has a drastic effect on the numbers of wild type recombinants, since the short 

 lengths provided by points of attack 2 and 3 will contain mutant portions of 

 the recipient site. It is these attack points that normally contribvite most 

 heavily to the formation of wild-type recombinants when the recipient cell 

 contains a point mutation. 



