In a DNA molecule, the distance between two bases is
2 nm/20 A
0.2 nm/2 A
3.4 nm/34 A
0.34 nm/3.4 A
One complete turn of the helix is 3.4 nm and has 10 base pairs. The bases face the interior of the double helix and are attacked at the 0.34 nm (3.4 A°) position.