The Double Helix

Two antiparallel strands wound around one axis, held together by paired bases, ten and a half pairs to every turn.

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Hover a base pair to read it

Anatomy

1
5′ → 3′ direction
The strands are antiparallel. The teal strand runs 5′→3′ along the axis; its red partner runs 5′→3′ the opposite way. The small chevrons show each strand's direction.
2
Sugar–phosphate backbone
Each strand's outer rail: deoxyribose sugars and phosphate groups joined by phosphodiester bonds. The phosphates give DNA its negative charge.
3
Major groove
The wide gap, about 2.2 nm across. The edges of the bases are most exposed here, so many DNA-binding proteins read the sequence through it.
4
Minor groove
The narrow gap, about 1.2 nm across. The grooves differ because the two backbones are not directly opposite each other around the axis.
5
Hydrogen bonds A=T · 2G≡C · 3
Adenine pairs with thymine through two hydrogen bonds, and guanine with cytosine through three. A purine always faces a pyrimidine, so every rung is the same width.

Measurements

10.5base pairs per turn

0.34 nmrise per base pair

≈ 3.6 nmlength of one turn

≈ 2 nmhelix diameter

Build a sequence

Top strand, read 5′ → 3′

Type A, T, G or C (2 to 40 bases). The partner strand is generated for you.

5′3′

3′5′

Legend

Adenine (A)purine · pairs with T

Thymine (T)pyrimidine · pairs with A

Guanine (G)purine · pairs with C

Cytosine (C)pyrimidine · pairs with G

Top strandPartner strandNewly built strands

The geometry follows idealised B-DNA: 10.5 bp per turn, 0.34 nm rise, about 2 nm across, with the two backbones offset by roughly 135° around the axis. Replication here is simplified: helicase, primase and polymerase are left out, and both new strands are drawn growing in the same direction.