H2 Biology Gel Electrophoresis Practical: Micropipetting, Agarose Gels and Band Interpretation

Good technique is slow and controlled. Bubbles, splashes, and rushed loading make band interpretation weaker.

3 | Agarose Gel Setup

A typical method includes:

1. Place the gel in the tank with wells near the negative electrode.
2. Add enough buffer to cover the gel.
3. Load DNA ladder into one lane.
4. Load each sample into a separate lane.
5. Connect the power supply with correct polarity.
6. Run the gel until bands are sufficiently separated.
7. Visualise or interpret the band pattern from the provided image or data.

Paper 4 may give prepared data instead of asking students to run the full gel. The same interpretation skills still apply.

4 | Band Interpretation

Use these rules:

• The DNA ladder is the reference lane.
• Bands closer to the wells are larger fragments.
• Bands further from the wells are smaller fragments.
• Samples with bands at the same position likely contain fragments of similar size.
• A missing band may mean no DNA, failed loading, degraded sample, or a detection limit problem.
• A smear suggests degraded DNA, overloaded sample, poor gel quality, or running conditions that were not controlled.

5 | Paper 4 Data Handling

If the question asks for fragment size estimation, a common approach is:

1. Measure distance travelled by ladder bands from the well.
2. Match each distance to known fragment size.
3. Plot migration distance against log fragment size if instructed.
4. Use the calibration graph to estimate unknown fragment size.
5. Quote units and uncertainty sensibly.

Do not interpolate beyond the range of the ladder unless the question explicitly permits it.

6 | ACE Evaluation Bank

7 | How To Write A Practical Answer

Strong answers combine method, principle, and control:

• "Use a fresh micropipette tip for each sample to prevent cross-contamination between wells."
• "Place the wells near the negative electrode because DNA is negatively charged and moves toward the positive electrode."
• "Include a DNA ladder so the size of unknown fragments can be estimated."
• "If bands are smeared, the sample may be overloaded or DNA may be degraded, making fragment-size estimation unreliable."

8 | Related Guides

• H2 Biology DNA Replication, Transcription, and Translation Notes
• H2 Biology Practical Private Candidate Guide
• Serial Dilution vs Simple Dilution in H2 Biology

Sources

1. SEAB. Biology Higher 2 (Syllabus 9477) GCE Advanced Level 2026.

Contact us to learn more about our tailored packages and available pricing options.