Looking at DNA
The complete set of your gene instructions is made up of two sets of 3000 million chemical letters, or bases of DNA. This is too much to study at once: scientists first have to cut it up and sort the pieces according to their size. They can then look at the pieces in detail, a few at a time. This allows them to work out the order of the bases (A, C, T and G) that make up a piece of DNA.