Standard Operating Procedure: Agarose Gel Electrophoresis

This Standard Operating Procedure (SOP) outlines the standardized method for the separation of nucleic acids (DNA/RNA) based on size using agarose gel electrophoresis. Proper execution of these steps ensures high-resolution separation, reproducibility, and safety. All personnel must adhere to these guidelines to maintain laboratory integrity and protect against chemical hazards associated with ethidium bromide or alternative nucleic acid stains.

1. Preparation of Materials and Equipment

• Ensure all necessary buffers (TAE or TBE) are prepared at the correct concentration (1x working solution).
• Verify the availability of agarose powder, electrophoresis grade.
• Check the casting tray, comb, and electrophoresis chamber for cleanliness and integrity.
• Prepare the nucleic acid stain (e.g., SYBR Safe or Ethidium Bromide) according to safety protocols.
• Ensure the power supply is calibrated and functional.

2. Gel Casting

• Calculate the required percentage of agarose (e.g., 1.0% for standard DNA fragments).
• Mix the agarose powder with the running buffer in an Erlenmeyer flask.
• Heat the mixture in a microwave, ensuring the solution is clear and no undissolved "crystals" remain.
• Allow the solution to cool to approximately 55°C before adding the nucleic acid stain.
• Secure the casting tray, insert the comb at the appropriate end, and pour the solution.
• Allow the gel to solidify at room temperature for at least 30 minutes; do not move the tray during this period.

3. Sample Loading and Electrophoresis

• Place the solidified gel into the electrophoresis tank and cover with 1x running buffer until the gel is submerged by 2–3 mm.
• Prepare DNA samples by mixing with the appropriate loading dye (typically 5x or 6x).
• Carefully load the samples into the wells, keeping track of the order. Load a molecular weight ladder in the first or last well.
• Attach the lid, ensuring the electrodes are aligned (Black to Black/Negative, Red to Red/Positive; "Run to Red").
• Set the power supply to the recommended voltage (typically 80V–120V) and start the run.
• Monitor the migration of the dye front; stop the run before the dye front reaches the end of the gel.

4. Documentation and Cleanup

• Carefully remove the gel from the tray and transport it to the imaging system.
• Capture the image using the appropriate light source (UV or Blue Light) and filter.
• Dispose of the gel and buffer according to institutional hazardous waste guidelines.
• Rinse the gel tank and trays with deionized water and air dry.

Pro Tips & Pitfalls

• The "Crystal" Pitfall: If agarose is not fully dissolved, the gel will have a cloudy appearance and yield poor resolution. If you see undissolved particles, continue heating in 10-second bursts.
• Buffer Depletion: Running the gel at too high a voltage for too long can cause the buffer to overheat, leading to "smiling" bands or gel melting. Use lower voltages for high-resolution requirements.
• Loading Technique: Practice loading with a practice gel. If you puncture the bottom of the well with your pipette tip, the sample will leak out beneath the gel, resulting in loss of data.
• Pro Tip: Always record the voltage and time of the run in your lab notebook to ensure you can replicate the specific banding patterns in future experiments.

Frequently Asked Questions (FAQ)

Q: Why is my DNA ladder smeary or absent? A: This is usually caused by DNA degradation (nuclease contamination) or improper gel concentration. Ensure all reagents are nuclease-free and the gel percentage is appropriate for the expected size of your fragments.

Q: Can I reuse the electrophoresis buffer? A: You can reuse the buffer for a limited number of runs (usually 2–3), but ensure the polarity and concentration remain consistent. If the buffer turns yellow or loses its buffering capacity, discard it immediately.

Q: What should I do if the dye front moves in the wrong direction? A: Immediately stop the power supply. You have likely placed the gel in the tank backward or swapped the leads. Always verify that the wells are closest to the negative (black) electrode, as DNA is negatively charged and migrates toward the positive (red) electrode.