Gel electrophoresis is a medical laboratory technique used to separate DNA fragments by size. It helps scientists compare genetic samples, check PCR results, and study DNA linked to disease. The method matters in medicine because it supports genetic testing, infection tracking, forensic identification, and research on inherited conditions.
A gel electrophoresis chamber makes invisible molecules easier to analyze by turning their movement into a visible band pattern.
Understanding Medical Technology: Gel Electrophoresis
The gel is usually made from agarose, a jelly-like material purified from seaweed. When it cools, it forms a mesh full of tiny spaces. The concentration of agarose changes the size of those spaces.
A more concentrated gel has smaller spaces and gives better separation of short DNA pieces. A less concentrated gel has larger spaces, which helps very long pieces move through. This choice matters because a gel that is too dense can trap large fragments, while a gel that is too loose may blur small fragments together.
Before the electrical current is switched on, samples are mixed with loading dye and placed into small wells at one end of the gel. The dye makes the sample easier to see during loading and helps it sink into the well. The gel sits in a buffer solution that carries electric current and keeps conditions stable.
Voltage must be chosen carefully. Higher voltage makes DNA move faster, but too much heat can distort the gel and make bands spread out.
The electric field strength equals voltage divided by the distance between the electrodes. A longer run can improve separation, though fragments can run too far and leave the useful part of the gel.
After the run, the DNA must be stained because DNA is normally invisible. Some stains glow under ultraviolet or blue light after binding to DNA. A camera records the glowing bands.
Each band contains many DNA fragments of a similar length. A thick bright band usually means more DNA is present, though brightness is not a perfect measurement. Scientists compare the sample lanes with a DNA ladder.
The ladder works like a ruler because its bands have known numbers of base pairs. Bands that line up horizontally have travelled similar distances and are likely to have similar lengths.
Results need careful interpretation. A single clear band after a PCR test can suggest that the intended DNA region was copied. Several bands may mean that more than one DNA region was copied, or that the sample contains mixed DNA.
A missing band can result from too little DNA, damaged DNA, an error during PCR, or a problem with staining. Laboratories use positive controls, which should produce a known band pattern, and negative controls, which should contain no DNA target. These checks help reveal contamination or failed reactions.
In medical work, a gel image is usually only one piece of evidence. Important decisions require confirmed results, good sample handling, and comparison with clinical information.
Key Facts
- DNA has a negatively charged phosphate backbone, so it moves toward the positive electrode in an electric field.
- Smaller DNA fragments travel farther through the gel because they pass more easily through its pores.
- Migration distance generally decreases as fragment length increases.
- Electric field strength can be described by E = V/d, where V is voltage and d is electrode separation.
- DNA fragment size is often measured in base pairs, written as bp.
- A DNA ladder contains fragments of known sizes and is used as a reference to estimate unknown fragment lengths.
Vocabulary
- Gel electrophoresis
- A laboratory method that separates charged molecules, such as DNA, by moving them through a gel using an electric field.
- Agarose gel
- A porous gel made from agarose that acts like a molecular sieve for separating DNA fragments.
- DNA ladder
- A mixture of DNA fragments with known sizes used to compare and estimate the sizes of sample fragments.
- Electrode
- A conductor connected to a power supply that creates the electric field in the electrophoresis chamber.
- Buffer solution
- A salt solution that carries electric current and keeps the pH stable during electrophoresis.
Common Mistakes to Avoid
- Loading DNA near the positive electrode is wrong because DNA is negatively charged and would run out of the gel instead of across it.
- Assuming larger DNA fragments move faster is wrong because larger fragments are slowed more by the gel pores.
- Forgetting to use a DNA ladder is wrong because there is no reliable reference for estimating fragment sizes from band position.
- Using too high a voltage is wrong because it can heat the gel, blur bands, and reduce separation quality.
Practice Questions
- 1 A gel chamber has electrodes 12 cm apart and a voltage of 120 V. Calculate the electric field strength using E = V/d.
- 2 A DNA ladder shows 100 bp, 500 bp, and 1000 bp bands. A sample band travels farther than the 500 bp band but not as far as the 100 bp band. Estimate whether the sample fragment is smaller or larger than 500 bp, and explain briefly.
- 3 Explain why DNA samples are placed in wells near the negative electrode before the power supply is turned on.