B-DNA — the double helix

Why the antiparallel double helix can be copied faithfully: the two sugar–phosphate backbones run in opposite directions, and each base pairs only with its complement (A–T, G–C). That rule means each strand carries the full recipe for the other.

What the animation shows

The helix rotates so you see its real 3-D depth; coloured rungs are the base pairs; when it “unzips”, each half is a template for a new partner strand — two identical copies.

Quantities

• Rise / bp = 0.34 nm — axial distance the helix climbs per base pair.
• bp / turn = 10.5 — base pairs in one full turn (B-form).
• Pitch = 3.57 nm/turn — bp/turn × rise; the length of one turn.
• Twist / bp = 34.3° — 360° ÷ 10.5; rotation added per base pair.
• Helix Ø ≈ 2.0 nm — width of the B-form duplex.

Sources

Watson & Crick, Nature 1953; standard B-form DNA parameters (Dickerson et al.).