DNA Double Helix

The idea

DNA is two sugar-phosphate backbones winding around a shared axis, joined rung by rung through paired bases: adenine with thymine (two hydrogen bonds), guanine with cytosine (three). The pairing rule is the molecule's masterstroke — because each base admits exactly one partner, either strand alone specifies the other completely. Pull the strands apart and each is a template for rebuilding its complement; that is how the molecule copies, and it is the mechanism Watson and Crick famously noted "has not escaped our notice". Pass this view a sequence and you see the rule enforced: colour the near strand's bases and the far strand's complement writes itself.

The geometry on screen is the real thing, scaled up about a hundred million times. B-DNA — the form your genome is overwhelmingly in — is a right-handed helix making a full turn every ~10.5 base pairs, rising 0.34 nanometres per rung. Those numbers are parameters here, not decoration, so the default helix is checkable against the textbook: count the rungs in one full twist.

Chirality is real in this render, and it matters: the mirror-image left-handed helix is a different molecule. A left-handed form of DNA does exist — Z-DNA, a strained variant that shows up transiently in stressed, GC-rich stretches — but the DNA of life's everyday business is right-handed, always.

What to look for

• Author by sequence — 'GATTACA' works — and click the rungs: each is split-coloured by base, with the Watson-Crick complement on the far strand. Every rung is either A·T or G·C; the render cannot show a mispair because the rule is built in.
• Check the handedness with your thumb: point it along the helix axis and the strands should climb in the direction your right hand's fingers curl. Most DNA drawn in logos and stock art fails this test.
• Count base pairs through one full turn of the default helix: ~10.5, the B-DNA number.
• Append ?walk=1 to the world URL and walk through the helix — the molecule at architecture scale.

Getting it right

• Left-handed DNA in an illustration is not a style choice; it is the wrong molecule. Once you have checked chirality here, you will spot the error everywhere.
• The ladder is not static. In a cell this helix is being unwound, read, copied, and repaired continuously, and it is wrapped around packaging proteins and supercoiled — the tidy free helix is the resting pose, not the working one.
• A-T and G-C are not equivalent rungs: the third hydrogen bond makes GC-rich DNA harder to pull apart — organisms living at high temperature run GC-rich, and the difference is why your PCR primers care about GC content.

Turn the knobs

• sequence authors the molecule from data; base_pairs mints an anonymous uniform helix when only the shape matters.
• handedness 'left' mints the Z-DNA mirror — put it next to the default and the difference is unmistakable.
• bp_per_turn and rise deform the geometry away from B-DNA: stretch the helix toward a ladder or wind it tight, and the canonical proportions start to look inevitable.