Gel electrophoresis is a fundamental laboratory technique used to separate large biological molecules, such as DNA, RNA, and proteins, based on their size and electrical charge. The separation process is driven by an electrical force that must be precisely generated and maintained. The power supply serves as the central engine for this technique, providing the regulated electrical energy required to move the molecules through a specialized gel matrix.
Creating the Electric Field
The power supply’s primary function is to convert the standard alternating current (AC) from a wall outlet into a stable direct current (DC) that the electrophoresis system requires. This conversion is necessary because the separation relies on a consistent, unidirectional flow of electrical charge. Once converted, the power supply delivers this DC energy to the electrodes within the gel box, establishing a strong electric field across the entire gel medium.
Most biological macromolecules, like DNA and RNA, possess an inherent negative charge due to their phosphate backbones. The power supply establishes a negative electrode, or cathode, where the samples are loaded, and a positive electrode, or anode, at the opposite end. The negative charge of the molecules is repelled by the cathode and strongly attracted to the anode, causing them to migrate through the gel matrix.
The rate at which each molecule travels is determined by its size, with smaller fragments navigating the porous gel more quickly than larger ones. The electric field must be robust enough to overcome the resistance of the gel and the surrounding buffer solution, yet controlled enough to prevent excessive heat generation that could compromise the sample or the gel itself.
The Physical Circuit and Setup
The power supply unit connects to a specialized electrophoresis chamber, often called a gel box or tank, to complete the electrical circuit. This tank contains the gel submerged in an electrically conductive buffer solution. Within the tank, electrodes—typically made of chemically inert materials like platinum or carbon—are positioned at either end to conduct the current into the buffer.
The current flows from the power supply, through the connecting wires, into the electrodes, and then through the buffer and the gel matrix before returning to the power supply. The buffer solution is an active part of the electrical circuit, as its ions carry the electrical current, and it maintains a stable pH environment for preserving the charge and structure of the macromolecules during the separation.
High voltages are often used in electrophoresis, which introduces a safety element into the design of the physical setup. Modern power supplies and gel boxes include safety interlocks that immediately cut the power if the lid is opened while the system is running. These features protect the user from accidental contact with the exposed electrodes and the high-voltage current flowing through the buffer solution.
Operational Control of Voltage and Current
Beyond simply generating a current, the power supply acts as a sophisticated control mechanism, allowing the user to select specific operating parameters. Researchers typically choose between running the experiment at a constant voltage or a constant current, each offering different advantages and trade-offs.
Constant Voltage
Running at a constant voltage, a common practice for separating DNA in agarose gels, ensures the electrical potential across the gel remains fixed throughout the run. As the separation progresses, the resistance within the system may change, causing the current and the overall speed of migration to decrease slightly over time.
Constant Current
Alternatively, a constant current setting maintains a fixed rate of electrical flow, which is often preferred for protein separation techniques like SDS-PAGE. Maintaining a steady current ensures a more uniform migration rate, which can lead to sharper, better-resolved bands. However, as the resistance in the gel system naturally increases during the run, the voltage must increase to keep the current constant, leading to a significant increase in heat generation, known as Joule heating.
Joule heating is a major practical consideration, as excessive heat can melt the gel or cause the separated bands to become distorted and diffuse. Precise control over the voltage, current, and maximum power settings allows the researcher to optimize the run for speed, resolution, and sample integrity.