Type a sequence and the helix is rebuilt on the spot. Tap a base on the ladder below or on the model to select that base pair, then rewrite it with A, T, G, or C. Raise the unzip amount and the two strands come apart from one end.
Two nucleotide strands face each other in opposite directions (antiparallel) and wind into a right-handed double helix.
The outer backbone is a repeat of phosphate and sugar (deoxyribose), and the bases point inward.
The pairing is fixed: A with T is held by two hydrogen bonds, G with C by three. This is complementarity.
One turn of the helix spans about ten base pairs, roughly 3.4 nm, and the helix is about 2 nm across.
Once one strand is known, the other follows automatically. That is the basis of replication and transcription.
G and C hold on more tightly with their extra hydrogen bond, so DNA with a higher GC content is harder to separate with heat.