246 



ERNST FREESE 



are noiuially cniploycd. Experience has shown that the spontaneous 

 frequency of mutations is approximately the same for (Ufferent growth 

 con(htions usually used in a laboratory. Nevertheless, small but signifi- 

 cant differences may be found and sometimes have been recorded as 

 "mutagenic" or "anti-mutagenic" effect. 



Some spontaneous nnitations nuist come about l)y mistakes in the 

 normal duplication of DNA or RNA. Actually it is surj)rising how few 

 mistakes are made during this process. Transitions might come about 



H\„/N^ P H-Q^ C-H 



I /C-c /:-{ 



N— C N— H N C— H 



/ \ / \ / 



— C N— C C-N 



\ \ / \ 



^ N— H O C— 



Guanine 



Thymine 

 (rare end form) 



Fig. 15. Rare base pairs which could occur spontaneously and would give rise to 

 transitions. Only the cases are shown in which the pyrimidine has undergone the 

 tautomeric shift. The same base pairs could occur when a purine has undergone a 

 tautomeric shift or even when anv one of the bases has been ionized. 



by the tautomeric shift (or ionization) of one of the bases (Watson and 

 Crick, 1953) which lead to the mistaken base pairing between A-C and, 

 probably more frequently, G-T, as illustrated in Fig. 15. Transversions 

 might come about by the mistaken pairing between two purines or two 

 pyrimidines (Freese, 1959b). There are several such pairs possible that 



