RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 



63 



Let us now see what kind of quantitative results such a model 

 yields. We examine the simplest case— that of transformation of a 

 mutant cell by a nomial DNA homologue. Figure 8 is a diagram of the 

 recombination events in such a transformation. While a polarity of 

 the recombination process is assumed in the drawing, this is merely to 

 simplify the drawing and is not an essential feature of the model. Sev- 

 eral possible points of attack are shown. These may be supposed to 

 correspond to one or both sugar-phosphate bonds linking each nucleo- 

 tide into the chain. Each point of attack is followed by a return, and the 

 probable length of sequence involved is shown by the normal curve 

 drawn from each point of attack. Clearly, the farther the point of attack 

 is from the mutational site, the longer must be the donor sequence in- 

 cluded in the recombinant, in order to exclude the mutational site in 

 the recipient cell. However, any given point of attack gives rise to very 

 few recombinants in which a long donor sequence is present. Thus, 

 distant points of attack will yield few wild-type recombinants. If the 

 point of attack is closer to the recipient cell's mutated site, points of 

 return delimiting shorter sequences will be effective in yielding wild- 

 type recombinants. Finally, the point of attack just to the left of the 



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Figure 8. Diagram of a recombination model for transformation of a mutant 

 recipient cell by wild-type donor DNA. The black rectangle marks the posi- 

 tion of the mutation. Dotted arrows indicate different points of attack of the 

 donor sequence. Curves describe the frequency distribution of the lengths of 

 donor sequence determined by a return to the recipient cell's DNA. The 

 scale below the donor sequence corresponds to two sigmas of the distribu- 

 tion; for any point of attack to the left of two sigmas, there is a 95 per cent 

 or greater chance of the return occurring before exclusion of the mutant site. 



