36 



CELL HEREDITY 



N 



NH2 





C— H 



h/^^n^^~^n' 



H 



Adenine 



OH 



H/^^N^'^^N^ 



C— H 



H 



-^- Hypoxanthine 



HoN' 



OH 



^^ 



C 



II 



Guanine 



/ 



H 



OH' 



OH 



"^ \ 





C— H 



^nt/^\. 



H 



Xanthine 



NHc 



OH^ ^N' 



C— H 

 .C— H 



Cytosine 



OH' 



OH 



I 



w c 



^^ 



H 

 C— H 



N 



Uracil 



FIGURE 2.1. The changes in bases resulting from oxidative deamination caused by 

 treatment with nitrous acid. 



designated wild type, any heritable variation from it is called mutant. 

 In some mutation studies, the mutated elements have been localized in 

 particular regions of chromosomes and even within the DNA. There 

 are also instances of nonchromosomal inheritance and, although the 

 chemical basis of this kind of heredity has not been determined, it is 

 probably not DNA. The ability of RNA to carry hereditary information 

 as established by work with plant and animal viruses may represent a 

 general feature of cell heredity. 



The fact that RNA and DNA are both polynucleotides provides a 

 common denominator accounting for some similarities in their mutability. 

 For example, nitrous acid acts as a mutagen for both substances because, 

 by oxidative deamination, it substitutes an hydroxy for an amino group 

 and converts adenine into hypoxanthine, guanine into xanthine, and 

 cytosine into uracil (Figure 2.1). 



