Each base pair sits at axial position n·rise and angular position n·twist around the helix axis (n = 0..N−1). This "unrolled" view plots both backbones' lateral offset as R(n)·sin(θ) with opposite sign, so the axis runs left→right instead of orbiting a 3D camera.
| Form | bp/turn | rise/bp | twist/bp | major groove | minor groove |
|---|
| B-DNA | 10.5 | 0.34 nm | 34.3° | 2.2 nm (wide/shallow) | 1.2 nm (narrow/deep) |
| A-DNA | 11.0 | 0.26 nm | 32.7° | 1.4 nm (narrow/deep) | 2.7 nm (wide/shallow) |
| Z-DNA | 12.0 | 0.37 nm | −30° (left-handed) | ~0.5 nm (nearly flat, convex) | 1.8 nm (deep, narrow) |
The complementary strand is generated by the real Watson–Crick rule, never arbitrarily: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds). Typing or randomizing the top strand instantly regenerates the bottom one, and each rung is drawn with the correct bond count.
- B-DNA — the common right-handed form found in most cellular DNA.
- A-DNA — a more compact right-handed form (dehydrated conditions, RNA:DNA hybrids); groove asymmetry inverts relative to B-DNA.
- Z-DNA — a left-handed, zig-zag backbone form; note the reversed winding direction and near-absence of a distinct major groove.