base stacking
Ask what holds the two strands of DNA together and most people say 'the hydrogen bonds between the bases'. That is half the answer. The other half, often the larger half, is a quieter force called base stacking — the way the flat bases pile up on top of one another, like a neat stack of coins.
Inside the double helix the base pairs are flat plates lying roughly perpendicular to the axis, and consecutive pairs sit directly above one another, just a few tenths of a nanometre apart. Their flat, electron-rich faces attract one another through a mix of van der Waals forces and the way their electron clouds interact, while the hydrophobic effect pushes these oily, water-shy faces together to escape the surrounding water. This stacking, running the length of the helix, contributes a large share of the stability that keeps DNA wound up and rigid enough to hold its shape.
Base stacking matters because it corrects a common misconception. Hydrogen bonds get the headlines, but stacking energy is comparable to or greater than the hydrogen-bonding energy in holding the helix together. Stacking is also why DNA absorbs ultraviolet light less strongly when intact than when separated — when the strands come apart and the bases unstack, absorbance jumps, a change biologists exploit to watch DNA melt and re-form in real time.
Heat a DNA solution and watch its ultraviolet absorbance at 260 nanometres: as the strands separate and the bases unstack, absorbance rises sharply — the 'hyperchromic effect' — giving a direct read-out of melting.
Stacked like coins — the quiet force often stronger than the hydrogen bonds.
Do not credit hydrogen bonds alone for holding DNA together. Stacking and the hydrophobic effect contribute as much or more; the hydrogen bonds matter most for getting the right base across, not chiefly for raw stability.