Agarose Gel Electrophoresis for DNA Isolation and Analysis

Principle

• DNA has a uniform negative charge because of phosphate groups in its sugar-phosphate backbone.
• When an electric current is applied, DNA migrates through the gel towards the positive electrode (anode).
• The gel acts as a molecular sieve:
  • Small DNA fragments pass more easily through gel pores and move faster/farther.
  • Large DNA fragments move slowly and remain closer to the wells.
• Thus, DNA fragments are separated mainly on the basis of length in base pairs (bp).

Materials Required

1. Agarose powder and electrophoresis buffer such as TAE or TBE
2. Gel casting tray, comb, electrophoresis tank and power supply
3. DNA sample mixed with loading dye
4. DNA size marker or molecular-weight ladder
5. DNA staining dye, for example ethidium bromide or safer fluorescent dyes
6. UV or blue-light transilluminator for visualization

Procedure

1. Preparation of gel: Agarose is dissolved in buffer by heating and poured into a casting tray. A comb is placed to form wells.
2. Loading samples: After gel solidifies, it is placed in the tank containing buffer. DNA samples mixed with loading dye are loaded into wells. A DNA ladder is loaded in a separate well.
3. Electrophoresis: Electric current is applied. DNA migrates from the negative end near the wells towards the positive electrode.
4. Separation: Fragments resolve into distinct bands according to size. Smaller fragments travel farther than larger fragments.
5. Staining and visualization: DNA bands are stained and viewed under UV or blue light. The size of an unknown fragment is estimated by comparison with the DNA ladder.
6. Isolation of DNA: If a particular band is required, it is cut out from the gel with a clean scalpel. DNA is then recovered from the gel slice using a gel-extraction kit and used for cloning, sequencing, or further molecular analysis.

Applications

• Analysis of PCR products
• Checking restriction enzyme digestion
• Estimation of DNA fragment size
• Detection of mutations by altered fragment pattern
• DNA fingerprinting and forensic analysis
• Purification of a selected DNA fragment for cloning or sequencing
• Preparation for Southern blotting

Types of Gel Used

• Agarose gel: Used for larger DNA fragments with relatively larger size differences.
• Polyacrylamide gel: Has smaller pores and gives high resolution; can separate DNA fragments differing by even one nucleotide.

Result Interpretation