Agarose gel electrophoresis is a fundamental laboratory technique used to separate DNA fragments based on their size. When scientists need to analyze DNA after restriction digestion, PCR, or purification, they turn to gel electrophoresis to visualize the results.
How Agarose Gel Electrophoresis Works
The technique relies on the fact that DNA is negatively charged and will migrate toward a positive electrode when placed in an electric field.
- Preparing the Gel
Agarose powder is mixed with TAE or TBE buffer (typically 1× TAE or 0.5× TBE) and heated until dissolved. The molten agarose is poured into a casting tray with a comb inserted to create wells. As it cools, it solidifies into a porous gel matrix.
- Loading the Samples
The DNA samples are mixed with a loading dye containing glycerol (to make them sink) and tracking dyes (to monitor migration). The samples are pipetted into the wells of the gel. A DNA ladder—a mixture of fragments with known sizes—is loaded in the first well as a size reference.
- Electrophoresis
An electric current is applied across the gel. Since DNA is negatively charged, the fragments move through the gel toward the positive electrode. Smaller fragments navigate the gel pores easily and travel quickly, while larger fragments move more slowly. Over time, the fragments separate into distinct bands based on their size.
- Visualization
After electrophoresis, the gel is stained with a DNA-binding dye such as ethidium bromide or SYBR Safe. When placed under UV light, the dye fluoresces, revealing the DNA bands. By comparing the positions of the bands to the DNA ladder, scientists can determine the size of each fragment.
