Most people think that the hydrogen bonds are what hold DNA together. While they are important for the overall stability, other forces actually drive the two strands to form a helix.

Do you remember why oil forms droplets in water? Or why lipids form bilayers in water?

Nonpolar molecules want to phase separate from water.

The base part of a DNA monomer has some nonpolar ("oily") character (does it look a bit like benzene?).

Hilight electrostatics in a DNA duplex. Note that phosphates are typically shown in yellow and the bound oxygens in red. Together they form a phosphate group.

Notice that the phosphates are negatively charged. Charged groups want to interact with the water dipoles and so want to be out in the water as much as possible. The bases and sugars (at least parts of them), on the other hand, are a bit "oily" and want to be away from water. The DNA duplex achieves this!

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