The target fragments are separated under identical buffer, temperature and time duration but using different gel concentrations. For agarose, the gel concentration is defined in g/100 ml (Hjertén, 1962) while the pore size of a polyacrylamide gel can be controlled by the total amount of acrylamide present (%T, where T = total concentration of acrylamide and bisacrylamide monomer) and the degree of cross-linking (%C, where C = bisacrylamide concentration) (Rüchel, 1978). This procedure, in fact, resembles a sieving process where the gels act as a sieving medium, defining nucleic acids based on fragment size in response to an electric current. The gels can be casted in a variety of shapes, sizes, and porosities. The choice within these parameters depends primarily on the fragments to be separated.