What is DNA?

DNA is a long, helically shaped molecule that stores instructions for the development and functioning of an organism. The instructions are written in an alphabet of four chemical letters, and the order of those letters is what carries the information. Nearly all of your cells hold a complete copy, though each one uses only the parts it needs.

DNA is a chemical chain built from four kinds of building block, called bases and written A, C, G and T. They are the only part of the chain that varies from one link to the next, so their sequence is the message. A run of that sequence spelling out one usable instruction is called a gene. Almost every gene is an instruction for making one protein, and proteins are what do the physical work in a cell. The DNA never builds anything itself. Reading a gene means copying it, and a messenger molecule called mRNA carries that copy out to where proteins are assembled. The chain is bases from end to end, but only a small share of it spells out genes. Much of the rest controls when genes are read, and a good deal has no known function at all. Which genes get read differs from cell to cell, and that is why identical copies produce skin in one place and muscle in another.

DNA works like a reference library that never lends anything out. The shelves hold every instruction the organism might need. When something is required, one page gets copied and the copy is carried out to a workshop, where the object is actually built. The library stays intact, the copy is thrown away afterwards, and two workshops requesting different pages end up making completely different things.