Helix

Fig. 01 / B-DNA, right-handed 10.5 bp per turn · 0.34 nm rise · 2.0 nm wide

Long plate: purine · short plate: pyrimidine · ticks: hydrogen bonds

Strand 1, 5′→3′

The partner strand writes itself: A pairs with T, G pairs with C.

Anatomy

Hover or focus a feature to trace it on the figure.

Sugar–phosphate backbone
Deoxyribose sugars alternate with phosphates, joined by phosphodiester bonds. The charged phosphates face the water.
Major groove 2.2 nm
The wide gap between the backbones. Transcription factors read base edges here without opening the helix.
Minor groove 1.2 nm
The narrow gap on the sugar side of each pair. AT-rich runs narrow it further.
Base pairs, hydrogen bonds 0.34 nm
A–T share 2 hydrogen bonds, G–C share 3. Pairs stack 0.34 nm apart and turn 34.3° each.
5′ → 3′, antiparallel
Strand 1 runs 5′→3′ left to right, its partner right to left. Polymerase only builds 5′→3′.

Replication

The strands part and each templates a new partner. Solid wire is parental, dotted wire is new.

Parental strand New strand Marked base pair

Reading a rung

Every rung joins a two-ring purine to a one-ring pyrimidine, so all rungs span the same 2.0 nm.

A = Tadenine (purine) with thymine (pyrimidine), 2 hydrogen bonds

G ≡ Cguanine (purine) with cytosine (pyrimidine), 3 hydrogen bonds

Which side carries the long plate tells A from T; the tick count tells A–T from G–C. Geometry is B-form and drawn to scale; the plate split is stylised*.