Agarose Gel Electrophoresis
Agarose gel electrophoresis is how molecular biologists "see" DNA and RNA fragments. You load samples into a gel, apply a voltage, and watch nucleic acids migrate so that smaller fragments travel farther. Bands illuminated with a DNA-binding dye become a simple, visual size distribution, still one of the most useful QC checks in any cloning or PCR workflow.
Why molecules move
At neutral pH, DNA and RNA carry a negative charge on their phosphate backbone, so they migrate toward the positive electrode. Agarose forms a porous mesh; larger fragments thread through more slowly. Within the useful range of a given gel percentage, migration distance relates to the logarithm of fragment length for linear DNA, which is why a ladder of known sizes lets you interpolate unknowns.
Gel percentage and resolution
Lower percentage agarose (for example ~0.7–1%) resolves larger fragments better; higher percentage gels tighten resolution for small PCR products. Very large genomic DNA and very small oligos sit at opposite edges of what a standard slab gel can do well, pulsed-field or polyacrylamide methods take over outside that window.
Buffer systems (TAE or TBE are common) maintain pH and provide ions for current. Voltage is a compromise: higher voltage is faster but can heat the gel and reduce resolution or distort bands.
Loading, ladders, and dyes
Samples are mixed with loading dye for density (so they sink into wells) and tracking dyes that migrate at characteristic rates. A molecular-weight ladder in a neighbouring lane is essential for size estimates. Intercalating or groove-binding stains make bands visible under the appropriate light; modern stains are often safer than older ethidium bromide workflows, but all DNA-binding dyes deserve respect and proper disposal.
RNA gels and denaturing conditions are used when secondary structure would otherwise blur size interpretation, a reminder that "a band on a gel" always depends on the nucleic acid's conformation and the buffer chemistry.
What you learn from a gel
- Did PCR work? A band at the expected size is encouraging; multiple bands suggest non-specificity.
- Is the digest complete? Restriction digests should shift plasmid bands to predicted fragment sizes.
- Is RNA intact? Distinct ribosomal RNA bands (in eukaryotic total RNA) versus a low-molecular-weight smear tell different stories.
- Is the prep contaminated with genomic DNA? High-molecular-weight material in an RNA prep can be a warning.
Gels are qualitative to semi-quantitative. Band brightness scales roughly with mass, but staining saturation and imaging settings make densitometry approximate.
Practical tips that save afternoons
Include a no-template PCR control and run it on the same gel. Do not overload wells, smiling or smeared bands often mean too much DNA or too much salt. If a band is needed for cloning, minimise UV exposure when excising, because UV damages DNA. Document ladder identity and gel percentage in your notes; a photo without that context is hard to reinterpret months later.
Takeaway
Agarose gel electrophoresis separates nucleic acids by size in an electric field through a porous gel. With a ladder, appropriate agarose percentage, and sensible staining, it remains the fastest reality check for PCR, digests, and nucleic-acid integrity, a visual bridge between the bench and every dataset that depends on clean amplicons or intact RNA.