# Gel electrophoresis

A method to sort DNA pieces by size. DNA is negatively charged, so in an electric field it moves through a gel toward the positive end. Small pieces move faster and further.

Biology (university) · Part 05: Gene regulation and biotechnology · Card 7 of 8 · Last updated 2026-10-19

## Analogy

Running through a crowded forest: small runners get through quicker.

## In practice

Comparing the band patterns of DNA cut with restriction enzymes shows similar or different samples, as in DNA profiling.

## Practice

### Flashcards

- **Which DNA fragments travel furthest in a gel?** The smallest ones.

### Interview questions

**Explain the three steps of a PCR cycle and why the technique is so useful.**

- Denaturation separates strands
- Annealing lets primers bind
- Extension by heat-stable polymerase
- Number of copies doubles each cycle
- Makes tiny samples testable

## Questions people ask

### Why does DNA move through the gel?

The phosphate groups in DNA carry negative charge, so DNA is pulled toward the positive electrode when voltage is applied. The gel is a mesh that slows long pieces more than short ones. Because every base pair adds about the same charge, size alone sorts the fragments.

### How are DNA bands made visible?

The gel is stained with a dye that sticks to DNA and glows under ultraviolet or blue light, or the DNA is labelled in advance with a fluorescent tag. A ladder of fragments of known sizes is run beside the samples so that the length of each band can be estimated.

## Related concepts

- [Polymerase chain reaction](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-6)
- [Restriction enzyme](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-4)
- [Recombinant DNA](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-5)
- [CRISPR-Cas9](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-8)

Web page: https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-7
Study this card in the app: https://studyvisually.com/app/learn?category=biology-university&card=biology-university-part-5-concept-7