Gel Electrophoresis Lab Answer Key

Understanding the Gel Electrophoresis Lab

Gel electrophoresis is one of those standard lab techniques you run into constantly, whether you're working in molecular biology, forensics, or just taking an undergraduate bio class. You load samples into wells, run a current through the gel, and fragments separate by size. The results tell you roughly what you expected, mostly. Students usually need an answer key after running their gels to check whether their bands line up with the controls or the known standards. I've helped people grade these labs for years, and the quality of interpretation varies wildly. A standard answer key for a gel electrophoresis lab will typically lay out the expected banding patterns for each sample well. You'll see things like the DNA ladder lane showing bands at known base pair sizes, the positive control with the expected result, and the experimental samples with predicted fragment sizes based on your restriction digest or PCR product. Your job is to compare your actual gel image against these expectations and note where things match and where they diverge. Most answer keys ask you to estimate the size of unknown bands by interpolation from the ladder. Measure the distance each ladder band traveled from the well, plot that against the log of the known base pair sizes, draw a best-fit line, and then use your unknown band migration distance to back-calculate its approximate size. A decent linear fit on a semi-log plot gives you something within 5 to 10 percent accuracy, sometimes better if your gel ran cleanly. If your ladder looks smeared or compressed, throw out that data and rerun the gel. There is no workaround for a bad ladder.

I once had a student whose entire gel came out fine except the ladder lane was a uniform blur. She spent an hour trying to force the calculation anyway and got nonsensical fragment sizes like 2347 base pairs for a band that clearly sat between the 1000 and 1500 bp markers. The fix was straightforward: load a fresh ladder, run the gel again, and this time make sure the old agarose didn't recrystallize in the well area before pouring. The second run took about 35 minutes of electrophoresis time at 100 volts and gave clean, sharp bands across every lane. When checking your results against the Gel Electrophoresis Lab Answer Key, pay attention to the question about why certain bands might be missing or extra. A missing band usually means incomplete digestion if you did a restriction digest, primer dimers consuming your reagents in a PCR setup, or simply that the fragment ran off the bottom of the gel because it was too small. An unexpected extra band often points to nonspecific amplification, partial digestion products, or contamination. These are the details that separate a C from an A on the lab report.

Common Pitfalls and What They Actually Mean

Students frequently misinterpret smeared bands as a technique error when it is actually just an overloaded lane. Loading too much DNA into a single well causes band stretching and merging, especially in the smaller fragment range where resolution is already tighter. If your ladder bands look thick and distorted rather than sharp, you loaded too much. A typical loading volume for a 1x TAE gel with a standard DNA dye is about 500 nanograms per lane for the ladder and similar amounts for unknowns. Going past 1 microgram per lane is where things start to degrade visibly. Another thing that trips people up is the direction of migration. DNA is negatively charged due to its phosphate backbone, so it moves toward the positive electrode, which is the anode. If your bands ran upward out of the gel or didn't move from the well at all, you either ran the gel backward with reversed electrodes or the power supply wasn't delivering current. I checked one gel where the student had connected the black lead to the red port on the power supply. The bands had migrated completely off the top of the gel and into the buffer. Nothing you can do about that post hoc other than rerun it. The concentration of your agarose matters more than most lab manuals emphasize. A 1 percent gel resolves fragments in the 500 to 10,000 base pair range reasonably well, but if your fragments fall below 200 base pairs, you need at least 2 to 2.5 percent agarose or a polyacrylamide gel. Using a 1 percent gel for small fragments will give you almost no separation, and your answer key expectations will not match reality. This is not a minor detail. It changes the entire interpretability of the lane.

Buffer choice also plays a role that gets glossed over. TAE runs clearer bands with sharper resolution for longer electrophoresis times, but it has lower buffering capacity and can run down over extended runs. TBE gives better resolution for small fragments and holds up longer, but the borate ions can interact with some dyes and inhibit downstream enzymatic reactions if you plan to excise and purify bands. Most undergraduate labs use TAE because it is cheaper and simpler, and that is fine for basic size estimation purposes. If you are using an answer key to grade or self-grade a lab, remember that slight variations in band position are normal and expected. Gel composition, voltage, run time, and even room temperature affect migration speed. Two gels run under identical conditions can still show band shifts of 5 to 10 percent in position. That does not mean the experiment failed. It means electrophoresis has inherent variability, and your conclusion should account for that rather than treating every mismatch as an error.