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Someone asked me why DNA and RNA are acids. I am **not** an expert in chemistry but I did study the basics of organic chem and do dabble a bit so I wanted to share the answer here.
Simple answer: Anything that lowers pH is an acid, RNA and DNA lower pH, therefore it is an acid.
Complex answer:
Nucleic Acids are called acids because, well, they lower pH, as anything that is an acid would. pH is, in simplistic terms, the concentration (logarithmicly) of H+ ions in a solution.
A unrelated side note with acids that dont directly donate a H+, your Lewis Acids, they still increase H+ in an aqueous solution because it effects the balance of H+ and OH- dissociation of the water itself. As such the H+ measure is still accurate. However Nucleic acids are not a Lewis Acid, they are a Bronstead-Lowry acid, which means they directly can dissociate and provide the H+ ion directly in an aqueous solution. But I'll get to that.
The individual parts of any molecule can be either acidic and basic, but a molecule overall will usually be one or the other depending on which dominates, RNA is no different, there are three major components to RNA, I attached a picture to show them. The components are a phosphate group, a sugar, and a nitrogenous base (the part that encodes data, your citosine and guanine and shit). Here is the cool part, the word base when talking about the "bases" of a strand is specifically chosen as the the word because they are themselves bases (on their own they would raise pH). Similarly the phosphor group, is also acidic, this should be obvious by its similarity to phosphoric acid. It is a proton donor for the same reason phosphoric acid is a proton donor.
So the only question remaining is why does the acidic phosphor group dominate over the Nitrogenous Base? Well for starters the the phosphate group has a pkA of near 0 , the Nitrogenous Base has a pkB of around 9.8 (depends on the base), so already the phosphate group is going to dissociate more readily than the Nitrogenous Group. However RNA strands are actually far more acidic than the individual Nucleotides that compose them happen to be. The reason for this is that RNA folds back in on itself with the bases associating with each other much like two halves of a DNA strand would. This causes the bases to be on the interior of the molecule while leaving the phosphate groups all around the outside. Since the bases are not exposed to the aqueous solution they do not dissociate as readily as they otherwise would while the phosphate groups are free to dissociate. Thus the RNA exhibits significant acidic properties.
By the way the acidic nature of RNA and DNA is intentional and functionally important. It means that the pH of the solution can be adjusted to effect the charge on the nucleotides and thus move it around. In a neutral pH solution the phosphate groups will have a negative charge. This results in the phosphates pushing each other away. This in turn can cause them to line up on opposite sides with their bases facing each other as well as help to straighten out a strand's backbone.
