How does gel electrophoresis determine the size of nucleic acids?

Gel electrophoresis determines the size of nucleic acids primarily by tracking their migration rate through a gel. In this process, nucleic acids are loaded into a gel matrix and an electric current is applied, causing the nucleic acids to move. Smaller nucleic acid fragments migrate faster and travel further through the gel than larger fragments because of the gel's porous structure. By comparing the distance migrated by the sample fragments to a standard molecular weight ladder, which contains fragments of known sizes, researchers can accurately determine the size of the nucleic acids being analyzed.

The other methods mentioned, while they may play a role in different contexts, do not accurately describe the primary function of gel electrophoresis in this scenario. For instance, measuring electrical conductivity or color changes under UV light does not provide a direct measure of nucleic acid size, and analyzing molecular weight typically requires different techniques rather than gel electrophoresis itself.