In DNA profiling, which step involves separating DNA fragments by size using an electric current through a gel?

• A. PCR amplification
• B. Gel electrophoresis
• C. Cloning
• D. Centrifugation

Answer: (B)

Gel electrophoresis is the step in DNA profiling that separates DNA fragments by size using an electric current through a gel. After preparing the DNA, the fragments are placed in a gel made of agarose. When the electric field is applied, DNA moves toward the positive electrode because of its negatively charged phosphate backbone. The gel matrix slows larger fragments more than smaller ones, so smaller fragments travel farther. This creates a banding pattern that reflects fragment sizes and can be used to compare DNA profiles between samples. PCR amplification increases the amount of DNA but doesn’t separate fragments by size; cloning makes copies of DNA in bacteria; centrifugation separates by density, not by sizing in a gel. Gel electrophoresis is the method that achieves size-based separation.