Standard Operating Procedure: Agarose Gel Electrophoresis

Introduction

Agarose gel electrophoresis is a fundamental molecular biology technique used to separate DNA or RNA fragments based on their size by applying an electric field across a porous agarose matrix. This SOP outlines the standardized procedure for preparing the gel, loading samples, performing the run, and visualizing the results. Adherence to these protocols ensures reproducible, high-resolution separation while maintaining laboratory safety standards regarding chemical and electrical hazards.

Preparation and Safety

• Ensure all reagents (agarose, TAE/TBE buffer, DNA stain) are within expiration dates.
• Wear appropriate Personal Protective Equipment (PPE), including a lab coat, nitrile gloves, and safety goggles.
• Verify that the electrophoresis chamber and power supply are clean and free of buffer leaks.

Step-by-Step Checklist

Phase 1: Gel Preparation

• Determine the appropriate agarose percentage based on the size of DNA fragments (e.g., 0.8% for large fragments, 2.0% for small fragments).
• Measure the required mass of agarose powder and add to a clean Erlenmeyer flask.
• Add the appropriate volume of 1x TAE or TBE buffer to the flask.
• Heat the mixture in a microwave until fully transparent; swirl periodically to prevent boiling over.
• Cool the solution to approximately 50-60°C.
• Add the DNA stain (e.g., SYBR Safe or Ethidium Bromide) at the manufacturer-recommended concentration and swirl gently.
• Pour the solution into the casting tray with the comb positioned correctly.
• Allow the gel to solidify at room temperature for 30-45 minutes.

Phase 2: Sample Loading and Electrophoresis

• Place the solidified gel into the electrophoresis tank and submerge it in 1x running buffer (TAE/TBE).
• Carefully remove the comb to reveal the sample wells.
• Prepare DNA samples by mixing with a DNA loading dye (typically 5x or 6x stock).
• Load the molecular weight ladder (marker) into the first well.
• Load the prepared DNA samples into subsequent wells, ensuring accurate pipetting.
• Secure the lid of the tank and connect the electrodes to the power supply (Red/Anode to Red, Black/Cathode to Black).
• Set the voltage according to the gel size and desired speed (typically 80V–120V).
• Start the run and monitor until the dye front has migrated approximately 75-80% of the gel length.

Phase 3: Visualization and Cleanup

• Turn off the power supply and disconnect the leads.
• Carefully remove the gel from the tank and place it on a UV or Blue Light transilluminator.
• Capture the gel image using the digital documentation system.
• Dispose of the gel and contaminated consumables in the designated hazardous waste container.
• Rinse the gel tank and casting tray with deionized water and air dry.

Pro Tips & Pitfalls

• The "Smile" Effect: Running the gel at too high a voltage generates excess heat, causing the lanes to curve (the "smile" effect). If resolution is poor, decrease the voltage.
• Well Integrity: Use a steady hand when removing the comb to avoid tearing the walls of the wells, which will cause samples to leak into adjacent lanes.
• Buffer Depletion: Overused buffer loses its buffering capacity, leading to poor separation or migration anomalies. Always use fresh buffer for critical experiments.
• Melting Points: If the agarose is poured while too hot, the plastic casting tray may warp; if too cool, the gel will polymerize unevenly with lumps.

FAQ

Q: Why is my DNA ladder smeary rather than consisting of distinct bands? A: This is often caused by degradation of the DNA sample due to DNase contamination, or by running the gel at an excessively high voltage for the buffer type used.

Q: Can I reuse the electrophoresis buffer? A: You may reuse the buffer in the tank 2-3 times, but it is recommended to discard it if you notice a change in pH or if the run times become inconsistent.

Q: How do I choose between TAE and TBE buffers? A: TAE is generally better for downstream DNA recovery (extraction) applications, whereas TBE has higher buffering capacity and provides better resolution for smaller fragments or long electrophoresis runs.