Sharp bands usually indicate appropriate sample amount and running conditions. Smearing can come from degraded template, overloaded DNA, salt contamination, excessive voltage, or poor gel preparation.
Ladder choice
The ladder should match the expected product range. A ladder that is too broad or too sparse can make product size interpretation uncertain even when the PCR worked.
Loading amount
More DNA is not always better. Overloading can widen bands, increase background, and obscure nearby products. For diagnostic gels, the goal is interpretable signal, not maximum brightness.
Voltage and heat
High voltage can speed the run but may heat the gel and distort bands. If the gel warms noticeably or bands curve, reduce voltage or run longer.
Practical review questions
- Is the gel percentage appropriate for the expected product size?
- Was the running buffer fresh and correctly diluted?
- Was the sample mixed evenly with loading dye?
- Did the ladder separate cleanly?
- Is the band problem present in every lane or only one sample?
Interpretation
Do not diagnose PCR failure from one poor gel lane. First decide whether the gel system itself produced reliable ladder and control behavior.