MUTATION AS A CHEMICAL PROCESS 215 



There are two kinds of base replacements that can be made by pairing 

 mistakes. In the first case a purine may be replaced by another purine, 

 or a pyrimidine by another pyrimidine (transitions). In the second case 

 a purine may be replaced by a pyrimidine, and vice versa (transversions). 

 The consequences are quite different. In the first case the only possible 

 transitions of base pairs are: 



^-^ and ^-^ 



(Remember that the pairs A and T and G and C would, in these inverted 



T A C G 



positions, give a different code to the DNA because of their relation to 

 neighboring base pairs. ) In the second case, the following transversions 

 are possible: 



A^ T 

 T^ A 



11 11 

 C^G 

 G^C 



They can be achieved through the incorporation by mistake of a purine 

 in place of a pyrimidine, or vice versa. In the case illustrated in Figure 

 8.7, guanine is incorporated opposite adenine instead of thymine. The 

 second replication results in a mutant DNA where, in the site once oc- 

 cupied by the base pair A:T, we now have C:G. 



All of the transpositions of both types are illustrated by double arrows 

 because the mutations should be reversible. However, if some mutagen 

 acted to cause a replacement of a pyrimidine by a pvrimidine or a purine 

 by a purine, then it could induce the first but not the second kind of base 

 change, and it could not reverse a mutation- of the second type. For 

 example, the change T: A to C :G, shown in the first illustration of Figure 

 8.6, could be both induced and reversed by such a mutagen, but the 



FIGURE 8.7. Example of a mu- 

 tation resulting from the in- 

 corporation by mistake of purine 

 in place of a pyrimidine during 

 the replication of DNA. 



A-T A-T A-C A-T 



A-T G-C 



Mutant 



