Use Play to build DNA from a single nucleotide up to the full double helix, or Step to advance one stage at a time. Drag to rotate the scene and click any structure — backbone, a base, a base pair, or a groove — to learn what it is.
Almost every cell in your body carries a complete copy of your DNA — the molecule that stores the instructions for building and running you. Stretched out, the DNA in one human cell is about $2\,\mathrm{m}$ long, yet it folds into a nucleus a few micrometres across. To understand how it stores information, we first need to see what it is actually made of.
DNA is a chain of nucleotides. Each nucleotide is a phosphate, a deoxyribose sugar, and one of four bases: adenine ($A$), thymine ($T$), guanine ($G$), or cytosine ($C$). The sugars and phosphates link into a backbone, and the bases face inward. The crucial rule is complementary base pairing: $A$ pairs only with $T$, and $G$ pairs only with $C$. Erwin Chargaff noticed this as a number pattern — in any DNA, the amount of $A=T$ and $G\equiv C$ (here $\equiv$ marks the three-hydrogen-bond pair). Because $A$ and $G$ are large two-ring purines and $T$ and $C$ are small one-ring pyrimidines, every rung is one of each, so the ladder stays a constant width.
The two strands run in opposite directions: one points $5'\!\to\!3'$ while its partner points $3'\!\to\!5'$ — they are antiparallel. The whole ladder then twists into a right-handed double helix, which lets the flat bases stack like coins and hides them from water. The numbers for B-DNA: about $10$ base pairs per turn, a rise of $0.34\,\mathrm{nm}$ per pair, a full turn every $3.4\,\mathrm{nm}$, and a width of about $2\,\mathrm{nm}$. The offset between the backbones leaves a wide major groove and a narrow minor groove where proteins read the code.
Press Play to assemble DNA from a single nucleotide up to the full helix. Then drag the Twist slider down to $0\%$: the helix unwinds into a flat ladder so you can read every base pair, then back up to $100\%$ to re-form B-DNA. Toggle Hydrogen bonds to see the two-bond $A\!-\!T$ and three-bond $G\!-\!C$ rungs, and click the major and minor groove labels to find where proteins dock.
| Stage | What appears | Key point |
|---|---|---|
| 1. Nucleotide | Phosphate + deoxyribose sugar + one base | The repeating building block |
| 2. Four bases | A, T, G, C colour-coded | Purines (A, G) vs pyrimidines (T, C) |
| 3. Base pairing | A–T and G–C rungs with H-bonds | A=T (2 bonds), G≡C (3 bonds) |
| 4. Antiparallel strands | 5′→3′ against 3′→5′ | Opposite directions, a flat ladder |
| 5. The twist | Ladder winds into a helix | ~10 bp/turn, right-handed |
| 6. Grooves | Major & minor grooves appear | Where proteins read the bases |
The model uses the standard textbook geometry of B-form DNA, the form found in cells under normal conditions. Stacked base pairs sit $0.34\,\mathrm{nm}$ apart, the helix completes one right-handed turn every $\sim 10$ base pairs ($3.4\,\mathrm{nm}$), and the duplex is about $2\,\mathrm{nm}$ across. The $A\!-\!T$ pair is held by two hydrogen bonds and $G\!-\!C$ by three, so DNA that is rich in $G$ and $C$ is slightly harder to pull apart — a real effect exploited every time DNA is heated to "melt" it apart.
Because $A$ only ever pairs with $T$ and $G$ only with $C$, the two strands carry the same information twice, in mirror form. Read one strand and you know the other exactly. That redundancy is what makes faithful copying possible: the cell simply unzips the helix and builds a fresh partner on each old strand, turning one molecule into two identical ones. Structure here is function — the shape of DNA is the reason heredity works.
DNA is a long polymer built from repeating units called nucleotides. Each nucleotide has three parts: a phosphate group, a five-carbon sugar (deoxyribose), and one of four nitrogenous bases — adenine ($A$), thymine ($T$), guanine ($G$), or cytosine ($C$). The sugars and phosphates link into a backbone, and the bases stick out and pair across the middle.
Key takeaway: DNA is a chain of nucleotides, each a phosphate, a deoxyribose sugar, and one of four bases A, T, G, or C.The bases pair in a fixed, complementary way: adenine always pairs with thymine ($A\!-\!T$), and guanine always pairs with cytosine ($G\!-\!C$). $A$ and $G$ are purines (two rings); $T$ and $C$ are pyrimidines (one ring), so every pair is one purine plus one pyrimidine, keeping the helix a constant width. $A\!-\!T$ is held by two hydrogen bonds and $G\!-\!C$ by three.
Key takeaway: A pairs with T, G pairs with C — a purine always with a pyrimidine, held together by hydrogen bonds.Each DNA strand has a direction set by its sugar-phosphate backbone, with a $5'$ end and a $3'$ end. In the double helix the two strands run in opposite directions — one goes $5'\!\to\!3'$ while its partner goes $3'\!\to\!5'$, like two one-way streets pointing opposite ways. This antiparallel arrangement is required for the bases to line up and pair correctly, and it matters during replication.
Key takeaway: the two strands run in opposite directions (5′→3′ against 3′→5′), which is what antiparallel means.Drawn flat, DNA looks like a ladder: two backbones as the rails and base pairs as the rungs. In reality the whole ladder twists into a right-handed spiral — a double helix — because that lets the flat bases stack on top of each other, which is energetically favourable and shields them from water. One full turn is about $10$ base pairs and $3.4\,\mathrm{nm}$ long.
Key takeaway: DNA is a twisted ladder; the twist lets the bases stack, stabilising it into a right-handed double helix.Because the two backbones are not exactly opposite each other, the twist leaves two unequal gaps spiralling along the outside of the helix: a wider major groove and a narrower minor groove. These grooves are where proteins such as transcription factors and polymerases reach in to read the bases without unzipping the strands. The major groove exposes more chemical detail, so most DNA-reading proteins bind there.
Key takeaway: the helix has a wide major groove and a narrow minor groove, and proteins read the DNA by docking into these grooves.Information is stored in the order of the bases along a strand — the sequence is the genetic code. Because $A$ only pairs with $T$ and $G$ only with $C$, each strand is a template for the other: knowing one strand tells you the partner exactly. To copy DNA, the cell unzips the helix and builds a new complementary strand on each half, so one molecule becomes two identical ones.
Key takeaway: the base sequence stores the information, and complementary pairing means each strand can template an exact copy of the other.