Nucleoside analogs that cause mutation I'm confused with this explanation in my book: 2-aminopurine is incorporated into DNA in place of adenine but can pair with cytosine, so an AT pair becomes a CG pair. This sentence seems odd to me. It says that adenine is replaced with 2-aminopurine, but can pair with cytosine. And the AT pair turns to CG pair. Does it mean that 2-aminopurine substitute adenine, and then it works like cytosine to bind with guanine?? Because 'AT pair becomes CG'. But it says 'but can pair with cytosine', which one is going to bind with cytosine?? I've Googled on this but couldn't reach to a clear explanation about this. Can someone please explain to me how this analog works or rephrase the sentence in my book? Thank you. The book that I'm referring is Microbiology: an introduction 10th ed. By tortora, funke and case. Page 229.

If you feed bacteria 2-aminopurine then at some low rate it gets incorporated into into the DNA instead of adenine during DNA replication. In other words there will be a base pair between thymidine and 2-aminopurine.

Two things could happen:

1. DNA repair could detect the unusual base and attempt to correct. If the 2-aminopurine is excised then adenine will be inserted--no mutation. If, however the thymidine is excised then sometimes a cytidine will be inserted. This gets resolved below.
2. During the next cell division the replication machinery will sometimes incorporate a dC across from the 2-aminopurine. Then in the following cell-division a dG will be incorporated across from the C, so what used to be an A-T base pair has become a G-C base pair (eventually).