A typical slab gel electrophoresis apparatus. Multiple samples are run on each gel. DNA migrates towards the anode (positive electrode). The supporting matrix (gel) is agarose or polyacrylamide. DNA molecules can be visualized (and photographed) under ultraviolet light after staining with ethidium bromide. Alternatively, radioactive DNA molecules (for example, labeled with 32 P) can be visualized by exposing an X-ray film (autoradiography).