In the DNA double helix, what is the approximate diameter?

In the DNA double helix, the approximate diameter is 2 nanometers.

This familiar measurement describes the most common cellular form, B-DNA. Its width is about 20 angstroms, or 2.0 nanometers, measured across the helix. The two sugar-phosphate backbones lie on the outside, while paired bases stack inside and hold the two strands together through hydrogen bonding.

The double helix is not a perfectly rigid pipe. DNA can adopt several conformations, including A-DNA and Z-DNA, whose dimensions differ slightly. B-DNA is roughly 2 nanometers wide and makes one complete turn every 10 to 10.5 base pairs, with a helical pitch of about 3.4 nanometers.

The measurement became part of the standard picture of DNA after structural work in the early 1950s. James Watson and Francis Crick’s 1953 model drew on X-ray diffraction evidence from Rosalind Franklin, Raymond Gosling, Maurice Wilkins, and others. A frequent mistake is confusing diameter with length: one DNA molecule can be extraordinarily long when stretched, even though the helix itself remains only about 2 nanometers across.