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AGAROSE GEL ELECTROPHORESIS

Separation, Sizing, and Visualization of DNA Fragments

1. Aim

To separate, analyze, and visualize DNA fragments using agarose gel electrophoresis and staining with Ethidium Bromide under UV light.

2. Principle

Agarose gel electrophoresis is a widely used technique in molecular biology to separate DNA fragments based on their size.

DNA molecules carry a negative charge because of the phosphate groups in their backbone. When an electric field is applied across an agarose gel, DNA molecules migrate toward the positive electrode (anode).

Fig 1: Schematic representation of DNA migrating from the negative electrode to the positive electrode.

The agarose gel acts as a molecular sieve, allowing smaller DNA fragments to move faster than larger fragments.

After electrophoresis, DNA fragments are visualized using Ethidium Bromide, which intercalates between DNA bases and fluoresces under a UV Transilluminator.

3. Materials Required

Chemicals and Reagents

• Agarose powder
• 1X TAE or TBE buffer
• DNA sample
• DNA loading dye (contains glycerol to add density and a tracking dye)
• DNA ladder (molecular marker)
• Ethidium Bromide
• Distilled water

Equipment

• Electrophoresis tank & Power supply
• Gel casting tray & Comb
• Micropipette and tips
• Gel documentation system & UV Transilluminator

4. Preparation of Agarose Gel

Expert Note: Ensure the ends of the gel casting tray are properly sealed before pouring the gel to prevent leaks.

1. Weigh 1 g agarose powder.
2. Add agarose to 100 ml 1X TAE buffer to prepare a 1% agarose gel.
3. Heat the mixture in a microwave oven until the agarose dissolves completely.
4. Allow the solution to cool to about 50–60°C.
5. Add Ethidium Bromide (0.5 µg/ml) carefully and mix gently.
6. Seal the ends of the casting tray and pour the gel.
7. Insert the comb to create wells.
8. Allow the gel to solidify for 20–30 minutes.

5. Procedure

1. Place the solidified gel into the electrophoresis tank.
2. Add 1X TAE buffer until the gel is completely submerged.
3. Carefully remove the comb from the gel.
4. Load the samples into wells using a micropipette.
5. Connect the electrophoresis unit to the power supply.
6. Run electrophoresis at 80–120 V for 30–45 minutes.
7. When the dye front reaches about 75% of the gel, stop the electrophoresis.

6. Visualization & Results

To measure the exact size of your DNA fragments, you compare your sample bands against the DNA Ladder lane. The ladder acts like a ruler.

Fig 2: Simulated view of glowing DNA bands. Notice how smaller fragments migrate further down the gel.

7. Troubleshooting Common Errors

Observation	Possible Cause
Smeared DNA Bands	DNA degradation by nucleases, too much DNA loaded, or voltage too high.
No Bands Visible	Forgot to add Ethidium Bromide, DNA concentration too low, or DNA ran off the end of the gel.
Gel Melting During Run	Voltage applied is too high or incorrect running buffer was used.

8. Advantages & Limitations

Advantages

• Simple to perform and cost-effective.
• Non-destructive (DNA can be extracted later).
• Easily handles a wide range of DNA sizes.

Limitations

• Poor resolution for very tiny DNA fragments (use PAGE instead).
• Ethidium Bromide is highly toxic/mutagenic.
• Low quantitative accuracy.

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9. Important Viva Questions & Answers

Q1. Why does DNA migrate towards the anode during electrophoresis?

DNA has a sugar-phosphate backbone. The phosphate groups give DNA a net negative charge. Because opposite charges attract, the negatively charged DNA moves towards the positive electrode (anode).

Q2. What is the role of Ethidium Bromide (EtBr)?

EtBr acts as an intercalating agent. It slides between the nitrogenous base pairs of the DNA double helix. When exposed to UV light, it fluoresces strongly, making the invisible DNA bands visible to the naked eye.

Q3. Why is glycerol added to the DNA loading dye?

Glycerol increases the density of the DNA sample. This ensures that when the sample is pipetted into the well, it sinks heavily to the bottom rather than floating away and dissolving into the running buffer.

Q4. What is the purpose of the TAE or TBE running buffer?

The buffer serves two main purposes: it provides the necessary ions to conduct the electrical current through the gel, and it maintains a stable pH so the DNA does not denature during the run.

Q5. How does the concentration of agarose affect DNA separation?

Agarose creates a porous molecular matrix. A higher concentration of agarose (e.g., 2%) creates smaller pores, which is ideal for separating very small DNA fragments. A lower concentration (e.g., 0.8%) creates larger pores, ideal for large DNA fragments.

Q6. What is a "DNA Ladder" and why is it loaded in the first well?

A DNA ladder is a mixture of DNA fragments of known sizes (measured in base pairs, or bp). It acts as a ruler. By comparing the distance your sample traveled against the ladder, you can accurately estimate the size of your DNA fragments.

Q7. Why do we add tracking dyes like Bromophenol Blue?

Because DNA is invisible to the naked eye, tracking dyes are added to monitor the progress of the electrophoresis. The dye migrates ahead of the DNA, letting the researcher know exactly when to turn off the power supply before the DNA runs off the edge of the gel.