Two strands wound together
The shape of the molecule that holds the instructions for living things.
What you are looking at
The dots come together into two spirals that wind around each other, joined by cross-pieces, like a twisted ladder.
The idea
Picture a rope ladder, and twist it into a spiral. The two long ropes are the sides. The steps in between are pairs of small pieces called bases. There are four kinds, called A, C, G, and T. A only ever pairs with T, and C only ever pairs with G.
The order of the letters along the ladder is the information. It works like the letters in a book: the order spells out instructions for building and running a living thing.
This also makes copying easy. Unzip the ladder down the middle. Each half now has only half of each step. Because A always goes with T, and C with G, each half can build the missing side. You end up with two ladders that are the same as the first.
Where else you see it
- Every cell in your body holds a copy of your DNA.
- DNA tests and family-tree tests read the order of the letters.
- The same four letters are used by nearly every living thing, from bacteria to people.
Go deeper: the proper names and the math
The two strands run in opposite directions (antiparallel, 5′→3′ and 3′→5′). A–T pairs share two hydrogen bonds and G–C pairs share three, which makes G–C rich regions harder to pull apart. In the common B-form, the helix turns once every ≈10.5 base pairs, rises ≈0.34 nm per pair, and is about 2 nm wide. A human genome has roughly 3.1 billion base pairs.
The double-helix structure was worked out in 1953 by Watson and Crick, using X-ray diffraction images made by Rosalind Franklin and Raymond Gosling and data shared by Maurice Wilkins. Because each strand specifies its partner, replication is semi-conservative: each new molecule keeps one old strand and builds one new one.