How long is the Watson-Crick DNA double helix's pitch in angstroms?

The Watson–Crick DNA double helix has a pitch of 34 angstroms. In structural biology, pitch means the distance along the helix’s axis required for one complete turn.

The familiar B-DNA model described by Watson and Crick is a right-handed double helix. Its two strands wind around a common axis, while complementary bases pair inside the structure: adenine with thymine and guanine with cytosine. A 34-angstrom pitch corresponds to roughly 10 base pairs per turn, with each base pair advancing about 3.4 angstroms along the axis.

An angstrom is a very small unit of length equal to 10⁻¹⁰ metre, or 0.1 nanometre. Thus, 34 angstroms is 3.4 nanometres. The figure is a characteristic approximate measurement for canonical B-DNA, not a universal value for every nucleic-acid helix.

Readers sometimes confuse pitch with the helix’s width. B-DNA is about 20–23 angstroms wide, whereas 34 angstroms describes the height of one full twist. Other forms, such as A-DNA and Z-DNA, have different geometries and dimensions.