Running gels and actually reading them is where most people lose their minds.
I spent three years in a forensic lab running STR profiles. The equipment rarely failed. The samples did. I have seen more samples degrade from improper collection than from any technical error in the electrophoresis run itself. That is a fact you do not find in the protocol manuals. The method separates DNA fragments by size using an electric field through a matrix. You load your samples into wells, apply voltage, and the negatively charged phosphate backbone pulls the fragments toward the anode. Smaller fragments move faster through the pores. Larger fragments lag behind. That is the entire physics behind it. What makes this technique useful for fingerprinting is that the fragment sizes correspond to specific genetic loci. When you amplify those regions with PCR and run them through the gel, the pattern of bands becomes your profile. Two samples from the same person produce identical band patterns. Two unrelated people will differ at multiple loci. The discrimination power depends entirely on how many loci you are examining.
The Setup Most People Get Wrong
The gel concentration is the first place things go sideways. Agarose percentage matters more than people admit. A 2 percent gel resolves small fragments well but chokes larger ones. A 0.8 percent gel lets big fragments through but blurs the differences between similarly sized pieces. For STR analysis, you are usually looking at fragments between 100 and 400 base pairs. A 3 percent gel, or a pre-cast polyacrylamide system, gives you the resolution you actually need. I learned this the hard way when a colleague ran a 1 percent gel on a degraded sample and could not distinguish between alleles that differed by only four base pairs. The bands smeared together into an indecipherable mess. We re-ran the samples on a 4 percent denaturing polyacrylamide gel and got clean separation. The sample was fine. The gel was the problem.
Running the Electrophoresis
Mix your agarose with TAE or TBE buffer. Do not use water. The ions in the buffer conduct current and maintain pH. Boil the mixture until the agarose is fully dissolved. Cool it to about 60 degrees Celsius before adding ethidium bromide or a safer alternative like SYBR Safe. Pour it into the casting tray with the comb in place. Let it set for 20 to 30 minutes. Remove the comb carefully. Place the gel in the electrophoresis chamber and add running buffer until the wells are covered by about 2 millimeters. Prepare your samples. Mix your PCR product with loading dye. Load each sample into a well. Load a DNA ladder into at least one well, preferably two, so you can bracket the sizes. Close the lid. Connect the leads. Anode is red, cathode is black. DNA is negative, so it moves toward the red end. Run at 100 volts for about 45 minutes. Watch for bubbles at the electrodes. If you see none, your contacts are poor. If you see excessive bubbling, your voltage is too high and your gel is heating up.
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Reading the Result
Remove the gel. Place it on a UV transilluminator or a blue light imager, depending on your stain. DNA binds the intercalating dye and fluoresces. Each band represents a population of fragments of the same size. Count the bands. Heterozygous loci show two bands. Homozygous loci show one band, though it may be brighter. Compare your sample bands to the ladder to estimate fragment sizes. The ladder is your ruler. Without it, you have no idea what size fragments you are looking at. Some ladders mark in base pairs. Some mark in kilobases. Make sure you know which one you have before you start interpreting anything.
Common Problems and What They Mean
Bands that curve upward at the edges indicate uneven cooling or a gel that is too thin in the middle. The current density is higher at the edges, so fragments move faster there. Use a thicker gel, or run at lower voltage. Smearing instead of discrete bands usually means you have too much DNA in the well, or your sample is degraded. Load less sample, or re-purify the DNA. Faint bands mean insufficient DNA or over-run. Stop the electrophoresis earlier, or increase your starting material. I encountered a specific issue once with a forensic sample where the alleles appeared to drop out at one locus. The band was simply too faint to see. The template DNA was present, but it was at a very low concentration, and the PCR had not amplified it efficiently. I re-amplified the sample with a different primer set that had better efficiency for low-copy templates, and the missing allele appeared. The gel was fine. The primer mismatch was the problem.
When This Method Fails Completely
Gel electrophoresis has limits. It cannot resolve fragments that differ by only one or two base pairs in a complex mixture. It requires relatively pure DNA. Contaminants like salts, proteins, or phenol interfere with migration and cause distortion. It is destructive. You cut the gel to excise a band, and you cannot reuse that sample for other tests without recovery procedures that introduce error. For routine fingerprinting in a forensic context, capillary electrophoresis has largely replaced slab gels. It offers higher resolution, automated detection, and digital data output. Slab gel electrophoresis is still useful for teaching, for quick checks, or when you do not have access to a capillary system. But do not pretend it is the gold standard anymore. It is a workhorse, not a precision instrument.
Practical Advice for Getting It Right
Make fresh running buffer. Old buffer has depleted ions and shifted pH, which changes migration rates. Calibrate your voltage. A power supply that reads 100 volts but delivers 120 will run your gel faster than you expect and generate more heat. Keep the gel cool. Heat distorts the matrix and broadens bands. Run a pilot lane with a known sample before you load your precious specimens. If the pilot looks wrong, fix the problem before you waste your actual samples. Document everything. Record the agarose percentage, the buffer composition, the voltage, the run time, the ladder used, and the imaging settings. Six months from now, when you need to reproduce the result or defend it in court, you will be grateful you wrote it down. Memory is unreliable. Paperwork is not. The technique is straightforward. The execution is where experience matters. You will make mistakes. You will get smeared gels and ambiguous bands. You will learn from them. The goal is not perfection. The goal is reproducibility and honest interpretation of what the gel actually shows you.
