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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.

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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).
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  • $\begingroup$ Oh!! @mdperry, I've got it!!!! Thank you so much!! And I've 2 questions. I'm wondering if that cytosine form a complementary pair with guanine, then what about 2-aminopurine, since it's a semi conservative replication, we shouldn't leave 2-aminopurine hanging there correct? Would it pair with a new cytosine again? $\endgroup$
    – Theresa
    Oct 11, 2015 at 16:48
  • $\begingroup$ And another question. This tortora text said that 5-bromouracil would pair with cytosine (if it's not being fixed by DNA pol) but what the Brock's book said it actually binds with guanine. Which one is correct? Or 5-bromouracil actually capable of pairing with both of them? $\endgroup$
    – Theresa
    Oct 11, 2015 at 16:51
  • $\begingroup$ @Theresa Re: fate of nucleoside analogs incorporated into DNA, I don't actually know how long the modified base persists, but I have read papers where one entire strand was labeled, and the analogs could be detected 10 or 20 cell divisions later. If I understand correctly, the 2-AP can form base pairs with either dT or dC in subsequent rounds of DNA replication. $\endgroup$
    – mdperry
    Oct 13, 2015 at 20:08
  • $\begingroup$ Re: BUdR (bromouridine deoxyribose--that is what you are referring to?) I am not familiar with all the details, but off the top of my head, you would not expect to find U in DNA, only in RNA (ordinarily). Since T pairs with A in DNA, it follows that U pairs with A in RNA. In RNA double-stranded regions rU can also form base pairs with rG, as long as the Br atom does not disrupt this U=G pairing I would naively assume that dU (and by extension Br-dU) can base pair with dG. A dU=dC base pair would be quite unusual since they are both smaller, pyrimidines. $\endgroup$
    – mdperry
    Oct 13, 2015 at 20:12

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