V. MOLECULAR MECHANISM OF MUTATIONS 



217 



obtained by covering one of the two strands. Each of the classes of base 

 alteration could be further subdivided, because mutagens may prefer- 

 entially attack one or the other of the bases or base pairs. So far only 

 the specificity of agents inducing replacements is sufficiently understood 

 to warrant such a subdivision; for reasons that will become clear later 

 it is useful to distinguish "transitions" and "transversions" (Freese, 

 1959b) as shown in the tabulation. 



Original base 



Ultimate base 



Name of base 

 change 



Purine 

 Pyrimidine 

 Purine 

 Pyrimidine 



Other purine 

 Other pyrimidine 

 Any pjTimidine 

 Any purine 



Transition 

 Transition 

 Transversion 

 Transversion 



For double-stranded DNA the possible base ])air ciianges are the 

 following: 



In order to find out how many and which nucleotide pairs of DNA 

 have been altered in a mutation one would like ideally to make a 

 sequence analysis of both normal and mutant DNA or at least to 

 determine chemically that portion of DNA which has been changed. At 

 present this is not possible; instead we have to proceed indirectly by 

 comparing genetic and chemical observations on the effects of different 

 mutagens on DNA. 



III. Biochemistry of Mutagens and Theory of Base Alterations 



Apart from radiations there are many chemicals that show some 

 mutagenic effect. It would be impossible to mention all these agents. 

 Rather, I shall point out various categories and elaborate in more 

 detail the mutagenic action of those agents whose chemical effects are 

 better understood. 



In principle one can distinguish agents that affect only replicating 

 nucleic acid and others that exert their primaiy action on resting nucleic 

 acid, although the ultimate fixation of the mutations may also require 



' See list of abbreviations in footnote 2. 



