DNA is the stuff which makes genes and chromosomes. It is the ultimate in DIY instruction recording (like an old-fashioned cassette tape) for self-building of living organisms. Scientists know a great deal about this double-stranded molecule and its related single-strand 'messenger' RNA (ribonucleic acid) which acts, among other things, as a template for building proteins. The basic code is incredibly simple. There are just four chemicals (called bases) Adenine, Thymine, Guanine, Cytosine - A T G C. The code is built up from combinations of those four. A always links to T; G to C. So when the molecule uncoils, as it does to make copies of itself, each base seeks out and links to its new partner (A to T, G to C) - and two new molecules build up where there was just one before. The details of this are very complicated but you'll get the main idea of how DNA is the genetic code -- the genotype -- for how to build proteins which are the building blocks of bodies -- the phenotype. The DNA also contains instructions for how to assemble everything, how big to make it and where to put it. That's why my beak and your nose is on our faces - and not somewhere else on our bodies. Nobody yet understands how all this works in detail.