So hydrogen bonds, London dispersion forces and so forth. The interior is hydrophobic, just like in proteins and the exterior is hydrophilic once again just like in proteins. And these two strands are complementary with respect to one another. Now, what about RNA molecules? Well, RNA molecules because they contain a slightly different sugar molecule they don't form the double helix. They exist predominantly as a single strand but they can also intertwine fold to form different types of tertiary and secondary structures.