Scientist loading agarose gel to separate DNA fragments.
© Science Museum / Science & Society Picture Library
Scientists use a technique called gel electrophoresis to sort DNA into size order, Great Ormond Street Hospital, London, May 2000. During gel electrophoresis, an electric field pulls the pieces of DNA through a jelly-like slab: a gel. Smaller pieces move through the gel quicker and travel further than larger pieces. Scientists treat the sorted DNA pieces with a chemical that makes them show up under ultraviolet light.