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.
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.