178 MUTATION AND PLANT BREEDING 



We assume that mutation represents a functional change in 

 base order. These X-ray-induced mutations must therefore have a 

 base order which looks different to the "reading mechanism" of the 

 cell. Perhaps the X-rays produce a truly different base (base pair?) 

 within the DNA or perhaps there is some sort of "transnucleotida- 

 tion" corresponding to the known transpeptidation induced by 

 agents such as X-rays which cause breaks in DNA chains. It is almost 

 necessary to suppose a mechanism for rare base substitution within 

 intact DNA molecules to account for mutation in the absence of 

 DNA duplication. 



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