Discussion
Which one of the following is not a DNA depurinating agent ?
Answer: C
The agent that is not a DNA depurinating agent is Dimethyl nitrosamine.
Alkylating agents such as ethyl ethanesulphonate, methyl ethanesulphonate (written 'ethane sulphonate' in the paper) and nitrosoguanidine react directly with DNA. They add alkyl groups to the N7 position of guanine and the N3 position of adenine; these alkylated purines weaken the glycosidic bond, and the base falls out, leaving an apurinic site.
Dimethylnitrosamine is different: it does not react with DNA as it is. It is a pro-mutagen that has to be activated by liver cytochrome P450 enzymes before it can methylate anything, which is why the key singles it out from the three direct-acting depurinating agents.
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