The instruction molecule
DNA is a long, thin molecule that almost every living cell carries inside it. Its job is simple to state and astonishing to think about: it stores the instructions for building and running a living thing. The same molecule sits in a bacterium, a tomato, and you — only the message written on it differs.
Think of DNA as a recipe book that a cell can read and copy. The reading produces the working parts of the cell; the copying lets one cell become two, passing the book on intact. The whole field of genetics grows out of understanding this one molecule and the way its message is read, copied, and inherited.
Where it lives
In animals, plants, and fungi, most DNA is packed inside the cell's nucleus, wound up into structures called chromosomes. A human cell holds about two metres of DNA folded into a space far smaller than a grain of dust. The complete set of DNA in a cell is its genome — the entire instruction book, every page included.
A gene is a stretch of DNA that carries one usable instruction — often the recipe for a single protein. Your genome contains roughly twenty thousand genes, but those genes take up only a small fraction of the whole; the rest does other things we'll meet later. For now, hold this picture: genome is the book, genes are the entries, DNA is the ink.
Why one molecule can do so much
DNA carries information the same way written language does: through a sequence. A book means different things depending on the order of its letters, even though every book uses the same alphabet. DNA uses an alphabet of just four letters, yet by arranging them in different orders along a molecule millions of letters long, it spells out the difference between a fern and a falcon.