What characteristic separates DNA fragments during gel electrophoresis?

DNA fragments are separated during gel electrophoresis primarily based on their size, with smaller fragments migrating faster through the gel matrix than larger ones. The gel, typically made of agarose or polyacrylamide, creates a molecular sieve that impedes the movement of larger DNA molecules. When an electric current is applied, negatively charged DNA moves toward the positive electrode, allowing for the separation of fragments according to their length. This size-based separation enables the analysis and comparison of DNA samples.