THE HEREDITARY ORDER: GENETIC INFORMATION 



teriophage is relatively low: one ov^er one hundred thousand 

 (10~5) to one over one hundred million (10~8) per generation. 

 A4any agents are known to increase the probability of mutation. 

 Among these "mutagenic agents" are X rays, ultraviolet rays, and 

 various chemicals, such as nitrogen mustard, organic peroxides, and 

 nitrous acid. When, for example, bacteria, or bacteriophage, or plant 

 viruses, or animal viruses, or naked viral nucleic acid are exposed to 

 nitrous acid; mutants are produced. The kinetics of the process 

 reveals that induction of mutations is a one-hit process. Nitrous 

 acid, when acting on nucleic acid, deaminates nucleic bases, that is, 

 takes off their NHo group. This means that the deamination of one 

 base is enough to produce one mutation. 



Let us consider, for example, an adenine/thymine pair. As a result 

 of deamination, adenine is transformed into hypoxanthine (Figure 

 9). The replication of the DNA double helix, as will be seen later, 



hypoxanthine 

 H Odenme thymine ( deam, noted oden.ne) thymine 



\_N N — H CH3 H 



< }-< >-< >-% /-" \ /"' 



' <._/ "-\_/"^" \/^-\ .-.Cy 



\ q/ \ / ^N r C N 



deamination \ / \ 



H 



hypoxanthine cytosine guanine cytosine 



H H 



\ 



/ 



C— N .0 H — N ,CH3 H 



^N^, > — C > — < ^C_N /O H-N /CH3 



/ C N— H N ';CH > N^ J:c C /C C 



^N Z \ — N ^c )^-" \ /" 



after one replication and tautomerization 



\-H C/ "^ 



after a second replication 



Figure 9. The Mutagenic Action of Nitrous Acid (from Robert 

 Lavalle). 



Nitrous acid deaminates adenine, which is thus converted into hypoxanthine. 

 At the first replication, hypoxanthine binds cytosine. At the second rephcation, 

 cytosine pairs with guanine. 



The original base pair adenine /thymine has been replaced bv the base pair 

 guanine/cytosine as a result of the action of the nitrous acid. 



[27] 



