15.4: DNA Agarose Gel Electrophoresis
Agarose gel electrophoresis is a laboratory technique commonly used to separate DNA fragments by size. However, it can also be used to isolate and purify DNA fragments using a gel extraction protocol.
Gel extraction follows five major steps: running gel electrophoresis to separate fragments, isolating the individual bands, extracting DNA from those bands, and removing the dye and salts from the extracted mixture to obtain pure DNA.
In cloning experiments, both the insert and vector DNA fragments are obtained after digestion with restriction endonucleases. These DNA fragments of varying sizes are mixed with contaminants, such as enzymes, salts, etc., that can inhibit the ligation reaction that follows. Gel extraction is therefore used to obtain pure DNA fragments before ligation.
To set up a gel for extraction, a lower percent (0.7-0.8%) agarose solution is used to ensure efficient migration of the DNA bands. In addition, a wide-combed gel cast is used to obtain thick DNA bands that are easy to isolate. Then, gel electrophoresis is performed at a lower voltage to prevent heating of the gel and damage to the DNA.
Once gel electrophoresis is complete, the ethidium bromide-stained DNA is identified by exposing the gel to long-wavelength UV for a short time. Short exposure to UV radiation prevents damage to DNA. This is followed by cutting out the desired band using a clean razor blade.
The isolated gel slice containing the DNA fragment of interest is then processed through one of the commercially available gel extraction kits. All of these kits follow the same basic principle. First, the agarose gel is dissolved by a buffer solution containing salts, such as sodium iodide. Next, DNA is bound to a column containing an anionic resin and washed a few times with a dilute alcoholic solution to remove impurities. The column is then dried by spinning in a centrifuge. The pure DNA can now be eluted with buffer or deionized water.