A technique for separating molecules of DNA, RNA, or proteins according to their size or electric charge. It is used, e.g., to analyse the fragments of DNA obtained following digestion of a DNA sample with restriction enzymes to detect mutations, or to create a DNA profile. In essence, the DNA mixture is placed at one end of a chamber containing semi-solid gel made of agarose or polyacrylamide. An electric field is applied and the negatively charged DNA fragments move through the gel to the positive pole at a speed depending on their size: smaller fragments move faster than larger ones. On completion, a fluorescent dye is applied to visualize the DNA fragments, which form a series of bands in lanes along the gel. Hence the number, size, and relative abundance of fragments in each DNA sample can be determined by comparing the bands with those obtained from a reference sample of known size composition. Protein samples require addition of the detergent SDS to polyacrylamide gel in order to give them the necessary negative charges, in the method termed SDS-PAGE (sodium dodecyl sulphate-polyacrylamide gel electrophoresis). In pulsed-field gel electrophoresis (PFGE) the voltage periodically switches between three different directions. This enables DNA fragments as large as 10 Mb to be resolved successfully, compared with the maximum of 50 kb for the standard technique. It is used, e.g., to analyse digests of entire chromosomes from bacterial cells to identify pathogenic subtypes. The whole process is integrated and miniaturized in a single instrument, with the gel contained in arrays of capillary tubes, samples applied using a micropipette, automatic detection of fluorescence, and computerized data analysis. Gel electrophoresis is an essential part of many techniques in molecular biology. See also southern blotting; western blotting.