Gel electropheresis

• Gel electrophoresis is used to separate DNA fragments according to their size. The DNA fragments are placed onto an agar gel and a voltage is applied across it.
• The resistance of the agar gel used in electrophoresis means that the larger the fragments, the more slowly they move. Therefore, over a period of time, smaller fragments will move further than large ones.
• If the DNA fragments in gel electrophoresis are labelled (e.g. with radioactive DNA probes) then their final positions in the gel can be identified by placing a sheet of X-Ray film over the agar gel for several hours. The radioactivity from each DNA fragment exposes the film and shows where the fragment is situated on the gel.
• Only DNA fragments up to 500 bases long can be sequenced using gel electrophoresis, therefore larger genes and whole genomes must be cut into smaller fragments by restriction endonucleases.