Under physiological conditions, B-DNA has approximately 10.5 base pairs per turn. This value describes the helical repeat: the number of paired bases that rotate around the DNA axis before the double helix completes one full 360-degree turn.
The often-taught value of 10 base pairs is a useful idealization based on early structural models. More detailed measurements of DNA in solution and under cellular-like conditions show that the twist is usually closer to 10.4–10.5 base pairs per turn. With an axial rise of about 0.34 nanometers per base pair, that repeat corresponds to a pitch near 3.4–3.6 nanometers.
The number is not an immutable whole-number property. DNA sequence, hydration, salt concentration, bound proteins, and torsional stress can alter local twist. In cells, DNA is also commonly negatively supercoiled, meaning its overall topology differs from relaxed B-DNA. That is why “10.5” is the best standard answer for physiological B-DNA, while 10 is commonly encountered in simplified diagrams. A-DNA is usually given as about 11 base pairs per turn, whereas Z-DNA has about 12.