How do nucleic acids interact with water?
I'll just use DNA as an example - the same is true for RNA, though. DNA has basically 3 main parts to its makeup; Phosphate group, ribose sugar, and nitrogenous base. Phosphate and ribose sugar are very hydrophillic, meaning they love water. The nitrogenous base is middle of the road - it can hydrogen bond, but it doesn't like water as much. So, if given the chance (base pairing), DNA will form a double helix (generally with 2 strands). RNA will do exactly the same thing, except using 1 strand. By forming the double helix, the DNA gets the nitrogenous bases (these are AGTC) out of water and into the center where they don't have to interact with water so much. The simple answer is that nucleic acids will form double helicies in water (if they can) so as to get their more hydrophobic parts out of water. That being said, the bases are not necessarily hydrophobic, but they aren't as hydrophillic as the other parts.