How can an electric field separate DNA fragments by length?

A mixture of DNA fragments can look like one clear liquid even when its pieces have different lengths. Gel electrophoresis spreads those fragments through a porous material so that differences become visible as separated bands.

DNA has a negatively charged backbone. An applied electric field drives it toward a positive electrode. The gel contains many small passages, making movement more difficult for longer linear fragments than for shorter ones under suitable conditions.

Fragments with similar lengths accumulate at similar positions. A stain or other detection method makes those groups visible. Researchers run a DNA ladder, containing known fragment sizes, alongside the sample to estimate the lengths of its bands.

The pattern reports separation under particular conditions rather than spelling out every DNA sequence. Two fragments can have the same length and different bases. Other techniques are needed when the question concerns the exact sequence or biological meaning of a fragment.