In forensic DNA profiling, what is capillary electrophoresis used for?

• A. Separating amplified DNA fragments by size to generate STR profiles.
• B. Sequencing entire genomes at high speed.
• C. Quantifying protein concentrations.
• D. Cloning DNA fragments.

Answer: (A)

Capillary electrophoresis is used to separate amplified DNA fragments by size to generate STR profiles. In forensic DNA profiling, PCR amplifies STR loci, producing fragments whose lengths reflect the number of repeats at each locus. The capillary instrument uses an electric field to move fluorescently labeled fragments through a thin capillary filled with a polymer; smaller fragments migrate faster, giving separation by size. As fragments pass the detector, fluorescence is recorded to create an electropherogram, where the peak positions correspond to specific STR allele lengths. From these sizes, a complete STR profile is constructed and can be compared across samples to help identify or exclude individuals. This method focuses on fragment sizing rather than reading nucleotide sequences, and it isn’t used for protein quantification or cloning.