The picture above shows a typical gel electrophoresis set up. The clear container in the center of the picture is called a gel electrophoresis chamber. It contains the agarose gel that will be loaded with genetic material, as well as a buffer solution. It is connected to a DC power supply via electrodes. This picture was taken at Paw Print Genetics laboratory in Spokane, Washington. Viney and Fenton (1998) defined the term electrophoresis as, “the migration of charged particles through a static medium under the action of an applied electric field (p. 76).
Just from this definition, it is clear that numerous physics concepts can be used to help explain why electrophoresis works. First, I will discuss charge and electric fields and how these principles are utilized in gel electrophoresis. This will be followed by a discussion of basic DC circuits and Ohm’s Law. This will introduce the concepts of voltage, current, resistance, and resistivity. As Cutnell & Johnson (2012) have explained, electric charge is an “intrinsic property” of both protons and electrons (c. 18. 1).
This charge can be either positive or negative and opposite charges attract each other, while like charges repel each other. This concept is crucial to understanding how gel electrophoresis works. As voltage is applied to the sytem, the positively charged ions (cations) that are present in solution travel toward the negative electrode in the upper part of the chamber. At the same time, negatively charged ions (anions) that are present in solution move twoard the positive electrode in the lower part of the chamber. This occurs because of the concept that opposite charges attract each other.
When a charge is around other charges, it can experience an electrostatic force. This force, like other forces, contributes to the net force on an object, which can alter its motion. This ties in directly to Newton’s second law, which states that , where m is the mass of the object and a is the object’s acceleration. For example, we could calculate the total net force on an electron, if we knew its acceleration. The mass of an electron is 9. 11×10-31 kg. If this electron is moving at 3. 5×108 m/s2 toward the positive electrode, then we get the equation N of force acting on the electron.
In the picture above, the power source is the gray box to the far right of the picture. This is what produces the electrical energy that is needed for a successful gel. Within this machine, positive and negative charges build up on their respective terminals, which creates an electric potential difference between the terminals. When this reaches a maximum, it is known as the electromotive force (emf, symbol: ? ) of the battery, which is measured in volts (V). For gel electrophoresis, an ? value between 50-1000 V is typically used and usually, it is on the lower side of this range.
The power supply is connected to two electrodes, which then connect to the chamber. This carries the current generated by the power supply to the chamber. Once to the chamber, the electrical current is carried through the gel via ions that are present in the buffer solution. Besides this, the buffer also helps to maintain the pH and the temperature of the system, so choosing the correct buffer is pertinent.
Because this set up utilizes a DC power supply, all of the current produced by the power supply moves in the same direction at all times (Cutnell & Johnson, Ch. 0. 1). This is important for gel electrophoresis so the molecules being tested only “run” in one direction, as opposed to an AC power source being utilized. If the charged molecules were able to run both up and down the gel, scientists would yield inaccurate results. Ohm’s Law states that voltage (V) is equal to the amount of current (I) multiplied by a constant resistance (R) in a DC circuit to give the equation .
In a gel electrophoresis set up, the gel itself acts as the resistor. Various materials have their own various resistivity values (? , which is an inherent property of the material and therefore remains constant as long as the material remains constant. Resistivity can be used to calculate the resistance of a material using the equation where L is the length of the material and A is the cross-sectional area of the material (Cutnell & Johnson, Ch. 20. 3). The DC circuit that is created by a gel electrophoresis set up can be illustrated by a simple circuit diagram as is shown below. Very early work with electrophoresis began in the early 19th century. Much of this work was done using Faraday’s laws of electrolysis.
During this time, scientists were fairly limited with what they were able to do with the technique, so most experiments focused on observing the “properties and behavior of small ions moving through aqueous solutions under the influence of an electric field” (History of electrophoresis, n. d. ). In the next century, the amount work beind done with electrophoresis expanded, mostly due to Arne Tiseliu. In the 1940s, Tiseliu developed the Tiselius apparatus at the California Institute of Technology.
At the time, this apparatus allowed various colleges and research labs to make large dvancements in the field of molecular biology and biochemistry. According to Kay (1988), the analysis that was now possible due to electrophoresis led to a gain in knowledge and understanding regarding enzyme composition, protein structure, protein diffusion, and amino acid structure, among others (p. 55). The problem that most scientists found with the Tiseliu apparatus is that it did not allow for discrete molecular visualization. This shortcoming led to the development of several other electrophoresis techniques.
This process really took off in the mid 1960s and as Chiang (2009) pointed out in his paper, does not follow a “neat, linear” timeline from the Tiselius apparatus to the set ups that are commonly found in labs today (pg. 518). This is because during this time period, numerous scientists were experimenting with numerous set ups, techniques, and machines, all of which may or may not have actually contributed to the gel electrophoresis set up that is most commonly utilized in labs today. This set up was developed in the early 1970s and not much has changed since then.
