Another example is the DNA double helix. The two anti-parallel strands of DNA are held together by A-T and G-C base pairs. While the selectivity of these base pair interactions is controlled mainly by hydrogen-bonding, a limiting factor in utilizing hydrogen bonds in the base pairs lies in the fact that in aqueous solvents, the energies of the complementary base pair interactions are small. This is overcome by using both pi-pi stacking and hydrophobic effects to stabilize the double helix.

Like the Roman god Janus, the glucose molecule and the bases in DNA, adenine, thymine, guanine and cytosine, have "two faces." They can utilize both noncovalent interactions and the hydrophobic effect to form dynamic complexes. Let's begin with the bases uracil and thymine.