The DNA and RNA differ by their sugar residues. The DNA has a pentose sugar that only differs by an -H for DNA and an -OH for the RNA. This adds more van der Waals to DNA making it form the double helix in an attempt to lower surface tension with water. The substitution of the base thymine of DNA instead of uracil for RNA has a similar effect. These differ only by an -CH3 for thymine and -H for uracil. The latter also imparts greater surface tension to the DNA causing it to pack as a double helix within water.