The Mechanical and Electrical Dynamics of Gel Electrophoresis - Ohm's Law
Ohm's Law: Relationships between electrical parameters
Ohm's law describes the relationship between the voltage, V, the current, I, and the resistance, R, in a DC circuit. A greater voltage produces a greater proportional current through a given resistor:
V = IR
A variation of Ohm's law describes how small changes in electrical current can produce large changes in the expenditure of power,P:
P = IV = I2R
Although the electrical current through the gel consists of both migrating buffer ions and sample molecules, the vast majority of the current is represented by the buffer ions. As voltage is applied, the cations in solution migrate toward the negative electrode in the upper chamber, and the anions (and negatively charged sample molecules) migrate toward the positive electrode in the lower chamber.
Several factors must be in balance for gel electrophoresis to proceed toward good results. Of major concern is the management of the heat generated by current flow. While small buffer ions, for example, might be more effective in preventing interactions among sample molecules, very mobile ions create a much more conductive buffer. Excessive current flow will be accompanied by excessive heat generation, which can cause convection currents, solution evaporation, or in the case of agarose electrophoresis the melting of the matrix itself. The management of heat is a major reason for the choice of bulky, organic ions, such as Tris base or glycine. (The following section treats the buffer system in more detail.)
Because temperature regulation is especially critical in both polyacrylamide and agarose electrophoresis of DNA and RNA, this type of electrophoresis is most often carried out under conditions of constant power (or constant current with agarose electrophoresis, for historical reasons). In denaturing PAGE electrophoresis, a relatively high temperature must be established and maintained to prevent the renaturation of sample molecules, but the temperature cannot be allowed to get too high. Constant power conditions (or constant current) provide the most precise regulation of heat generation. A small variation in buffer concentration can be managed under conditions of constant power but would lead to a total failure under conditions of constant voltage.
Constant voltage conditions, however, are generally employed in the SDS-PAGE electrophoresis of proteins. Here, the generally smaller apparatus has less difficulty exchanging heat with the environment, and sample denaturation is not nearly as temperature sensitive. In this scenario, the advantages of directly controlling the field strength via constant voltage conditions outweigh those of directly controlling the power generation.