B-DNA has about 10.5 base pairs in one complete turn of its double helix.
B-DNA is the common, right-handed form of DNA found under ordinary cellular conditions. Each successive base pair rotates roughly 34.3 degrees around the helix axis; after about 10.5 base pairs, the DNA has completed a 360-degree turn. The helix rises approximately 3.32 Å, or 0.332 nanometers, per base pair, giving a pitch of about 3.32 nanometers per turn.
A common mix-up is the simplified figure of 10 base pairs per turn. That number is often used in diagrams and basic biology explanations, while 10.5 is the more precise structural value for B-DNA in solution. The exact twist can vary slightly with sequence, temperature, salt concentration, and whether DNA is under torsional strain. This variability explains why scientific sources may write “10–10.5” or “10.4–10.5,” even though 10.5 is the standard quiz answer.