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. While slab agarose gels remain the standard for routine DNA analysis, capillary gel electrophoresis (CGE) offers an automated, high-throughput alternative with laser-induced fluorescence detection, widely used in forensic DNA profiling and DNA sequencing.
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.
Practical Gel Electrophoresis Protocol
Select the agarose percentage based on the expected fragment sizes: 0.7% for 0.8–10 kb, 1.0% for 0.5–5 kb, 1.5% for 0.2–3 kb, and 2.0% for 0.1–1 kb. Weigh the appropriate amount of agarose (e.g., 1 g for a 1% gel) and add 100 mL of 1× TAE buffer. Heat in a microwave until the agarose dissolves completely — swirl every 30 seconds to prevent superheating. Cool to ~60°C, add 5 µL of GelGreen or ethidium bromide (10 mg/mL), mix, and pour into a casting tray with the comb inserted. Allow to solidify for 20–30 minutes. Place the gel in the electrophoresis tank filled with 1× TAE. Mix 5 µL of each DNA sample with 1 µL of 6× loading dye (containing glycerol and bromophenol blue). Load the entire volume into a well. Load 5 µL of a 1 kb Plus DNA ladder in the first and last wells. Run at 5–10 V/cm (measured as distance between electrodes) — for a 10 cm gel, 80–100 V is typical. Run until the bromophenol blue dye front has migrated approximately two-thirds of the gel length (30–60 minutes). Visualize on a UV transilluminator (302 nm for ethidium bromide, 470 nm for GelGreen) and photograph. For downstream applications (gel extraction), use a long-wavelength UV (365 nm) to minimize DNA damage.
Real-World Application
After PCR amplifying a 1.5 kb gene fragment, 5 µL of product is run on a 1% agarose gel alongside a 1 kb ladder. A bright single band at 1.5 kb confirms successful amplification with no primer-dimer or nonspecific products. The gel is photographed, and the band is excised under UV for purification and cloning.
