V. MOLECULAR MECHANISM OF MUTATIONS 



231 



could then stop at this site, which would be merely lethal, or any one 

 of the four DNA bases could get incorporated, into the new comple- 

 mentary strand, or finally the gap could be left out and thus cause a 



transversions 



transition 



no base pair change 



Fig. 11. Base pair replacements which could be induced by the removal of a 

 guanine base from DNA. 



small deletion (Freese, 1959c). The consequences of the incorporation of 

 one of the four bases are illustrated in Fig. 11 for the case of a G 

 removal ; in this case one would expect both transitions and transversions. 



4. Exposure to High Temperatures 



Similar to the effect of DNA exposure to too low pH a temperature 

 increase above the "melting temperature" causes strand separation 

 (Doty et ah, 1959). Again, in addition to this effect heat apparently 

 causes some depurination of DNA (Greer and Zamenhof, 1962) and 

 hence must also induce chain breakage. Zamenhof and Greer (1958) 

 showed that heating to 60°C (below the melting temperature) is muta- 

 genic for the E. coli strain W6. Since the extent of these mutations has 

 not been genetically determined it is not known whether they are 

 "point mutations" or larger alterations. Temperature effects on the pro- 

 duction of mutations have also been found for Tradescantia (Sax, 1937) 

 and Drosophila (Plough, 1941). 



5. Alkylating Agents 



There are many agents that carry one, two, or more alkyl groups in a 

 reactive form; these are called mono-, bi-, or polyfunctional alkylating 

 agents. The various groups on a polyfunctional agent may either act 

 separately or they may cause the cross-linking of molecules. Figure 12 

 shows the chemical structure of the most commonly used classes of 

 alkylating agents. 



