Southern blot DNA analysis, illustration. Enzymes are used to fragment DNA (deoxyribonucleic acid). The fragments are separated in an electrophoresis gel (upper left). The separated fragments are 'blotted' (upper right) onto a nitrocellulose filter which is removed (lower left) and incubated (lower centre). The radioactive markers (red dots, lower centre) indicate the presence of specific genetic sequences. A sheet of photographic film is then laid over the filter. The radioactivity darkens the film (lower right), which reveals the position of genes in the DNA. This technique is widely used in research. For this artwork with labels, see C023/8900.