14.1.9.2 Deaminating agents e.g. HNO2

· Nitrous acid is a very potent mutagen, and acts directly on either replicating or nonreplicating DNA by oxidative deamination of adenine, guanine and cytosine (which contain –NH2 groups).

· Conversion of –NH2 groups to keto groups changes the hydrogen-bonding potential of the bases.

· Adenine is deaminated to hypoxanthine, which base-pairs with cytosine rather than thymine causing AT to GC transitions.

· Cytosine is converted to uracil, which base-pairs with adenine instead of guanine, causing GC to AT transitions.

· Guanine is converted to xanthine, which is not directly mutagenic, because it base-pairs with cytosine just like guanine.

 

 

Figure 14.10 Mutagenic action of nitrous acid. Oxidative deamination converts (a) adenine to hypoxanthine, causing AT → GC transitions; (b) cytosine to uracil, causing GC → AT transitions; and (c) guanine to xanthine, which is not directly mutagenic. The combined effects of nitrous acid on adenine and cytosine explain its ability to induce transitions in both directions AT↔ GC.

The combined effect of nitrous acid on adenine and cytosine explain its ability to induce transitions in both directions

Therefore, nitrous acid mutations can also be induced to revert with nitrous acid.

IDevice Icon Activity

Explain how nitrous acid causes mutation AT → GC transitions and AT → GC transitions