How to Read Gel Electrophoresis

••• the molecule of tryptophane image by Stanislav Pepeliaev from

Gel electrophoresis is the last of many steps in determining a DNA fingerprint, determining paternity or searching for a genetic marker for disease. The process takes samples of DNA that are cut into smaller pieces and runs an electric current through the gel to move the DNA pieces. When this process is completed and the gel is stained, different lines of DNA will appear and the size of those DNA samples determines the DNA fingerprint.

Reading the gel

    Place the stained gel on a UV transilluminator if it was stained with ethidium bromide. Place the stained gel on a white light box if it was stained with methlyene blue.

    Turn on the illuminator and identify the length of the DNA samples in the ladder lane.

    Compare the samples in the first lane to the closest DNA sample in the marker ladder. Extrapolate this information to give you an approximate length of the DNA samples in this lane.

    Repeat Step 3 for each different lane that has a DNA sample.

    Compare the lengths of the samples in each well to each other to find similarities.


    • Sometimes your DNA sample will not line up perfectly with a sample from the marker ladder, so you will have to make educated guesses in these cases. When comparing two different gels, it is important to use the same DNA marker ladder or at least to run them at the same voltage for the same length of time.


    • Ethidium bromide is exceptionally carcinogenic, so care should be taken during handling and during proper disposal.


About the Author

Matthew Williams has his Bachelor's degree in biology with a minor in chemistry and also holds his Master's degree in Secondary Education. While concurrently working on two more Master's degrees, he teachers advanced biology at the high school level full time. His major passion is music and he has played numerous instruments over the past 20 years, including piano, guitar, bass

Photo Credits

  • the molecule of tryptophane image by Stanislav Pepeliaev from

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