After digestion of DNA with a restriction enzyme (Chapter 50), it is usually necessary, for both preparative and analytical purposes, to separate and visualize the products. In most cases, where the products are between 200 and 20,000 bp long, this is achieved by agarose gel electrophoresis. Agarose is a linear polymer that is extracted from seaweed and sold as a white powder. The powder is melted in buffer and allowed to cool, whereby the agarose forms a gel by hydrogen bonding. The hardened matrix contains pores, the size of which depends on the concentration of agarose. The concentration of agarose is referred to as a percentage of agarose to volume of buffer (w/v), and agarose gels are normally in the range of 0.3 to 3%. Many different apparatus arrangements have been devised to run agarose gels; for example, they can be run horizontally or vertically, and the current can be conducted by wicks or the buffer solution. However, today, the "submarine" gel system is almost universally used. In this method, the agarose gel is formed on a supporting plate, and then the plate is submerged into a tank containing a suitable electrophoresis buffer. Wells are preformed in the agarose gel with the aid of a "comb" that is inserted into the cooling agarose before the agarose has gelled. Into these wells are loaded the sample to be analyzed, which has been mixed with a dense solution (a loading buffer) to ensure that the sample sinks into the wells.