Gel electrophoresis is a widely used technique for the analysis of nucleic acids and proteins. Most every molecular biology research laboratory routinely uses agarose gel electrophoresis for the preparation and analysis of DNA. We will be using agarose gel electrophoresis to determine the presence and size of PCR products.
Electrophoresis is a method of separating substances based on the rate of movement while under the influence of an electric field. Agarose is a polysaccharide purified from seaweed. An agarose gel is created by suspending dry agarose in a buffer solution, boiling until the solution becomes clear, and then pouring it into a casting tray and allowing it to cool. The result is a flexible gelatin-like slab. During electrophoresis, the gel is submersed in a chamber containing a buffer solution and a positive and negative electrode. The DNA to be analyzed is forced through the pores of the gel by the electrical current. Under an electrical field, DNA will move to the positive electrode (red) and away from the negative electrode (black). Several factors influence how fast the DNA moves, including; the strength of the electrical field, the concentration of agarose in the gel and most importantly, the size of the DNA molecules. Smaller DNA molecules move through the agarose faster than larger molecules. DNA itself is not visible within an agarose gel. The DNA will be visualized by the use of a dye that binds to DNA.
Purpose: To determine the presence or absence of PCR products and quantify the size (length of the DNA molecule) of the product.
6X Sample Loading Buffer
DNA ladder standard
Gel casting tray and combs
Pipette and tips
4.84 g Tris Base
1.14 ml Glacial Acetic Acid
2 ml 0.5M EDTA (pH 8.0)
- bring the total volume up to 1L with water
Add Tris base to ~900 ml H2O. Add acetic acid and EDTA to solution and mix. Pour mixture into 1 L graduated cylinder and add H2O to a total volume of 1 L. Note for convenience a concentrated stock of TAE buffer (either 10X or 50X) is often made ahead of time and diluted with water to 1X concentration prior to use.
6X Sample Loading Buffer
1 ml sterile H2O
1 ml Glycerol
enough bromophenol blue to make the buffer deep blue (~ 0.05 mg)
-for long term storage, keep sample loading buffer frozen.
QUIKView DNA Stain
25 ml WARDS QUIKView DNA Stain
475 ml warm water (50-55 C)
Agarose Gel Electrophoresis Protocol
Preparing the agarose gel
Measure 1.25 g Agarose powder and add it to a 500 ml flask Add 125 ml TAE Buffer to the flask. (the total gel volume well vary depending on the size of the casting tray) Melt the agarose in a microwave or hot water bath until the solution becomes clear. (if using a microwave, heat the solution for several short intervals - do not let the solution boil for long periods as it may boil out of the flask). Let the solution cool to about 50-55°C, swirling the flask occasionally to cool evenly. Seal the ends of the casting tray with two layers of tape. Place the combs in the gel casting tray.
Pour the melted agarose solution into the casting tray and let cool until it is solid (it should appear milky white). Carefully pull out the combs and remove the tape.
Place the gel in the electrophoresis chamber.
Add enough TAE Buffer so that there is about 2-3 mm of buffer over the gel.
Note gels can be made several days prior to use and sealed in plastic wrap (without combs). If the gel becomes excessively dry, allow it to rehydrate in the buffer within the gel box for a few minutes prior to loading samples.
Loading the gel
Add 6 l of 6X Sample Loading Buffer to each 25 l PCR reaction Record the order each...