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It is well known that the A in both DNA and RNA stands for acid, but where is the acid in chemical formula for the compound, and it is classified so based on what acid-base theory? Like Arrhenius, Bronsted-Lowry or Lewis.

Just to better explain my question, when I say DNA I am essencially thinking about nucleotides linked together, am I missing something in the structure that contains my answer or this is the way to go?

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2 Answers 2

The phosphate ester groups which connect the nucleotides contain one acidic proton at their OH group, and two at the end of each strand. As you can see here, here and in the picture below (with the groups marked turquoise), most of these groups are deprotonated depending on pH. They act as Brönsted-Lowry acids.

enter image description here

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Just to better explain my question, when I say DNA I am essencially thinking about nucleotides linked together, am I missing something in the structure that contains my answer or this is the way to go?

Yes, you do. *NA consist of nucleotides, linked by phosphodiester fragments. This leaves one hydroxyl of the phosphate fragment intact, so it can act as an acid.

Look closely:

enter image description here

There is a $\ce{P-O-}$ fragment, which means that there is a cation somewhere nearby. So, *NA are Brønsted acids, usually occuring as salts.

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