Which statement best describes the principle behind gel electrophoresis?

• A. DNA fragments are separated by size in a gel under an electric field
• B. DNA fragments are separated by charge but not by size
• C. DNA fragments are separated by sequence
• D. DNA molecules migrate through a gel matrix under an electric field, separating by size

Answer: (D)

DNA gel electrophoresis separates fragments by size as they migrate through a gel matrix under an electric field. The DNA backbone is highly negatively charged, so in the applied field it moves toward the positive electrode. The gel matrix acts like a sieve; its pores slow larger fragments much more than smaller ones. As a result, smaller pieces slip through the pores and travel farther, while bigger pieces lag, producing a size-based separation. The description that includes movement through a gel matrix under an electric field and emphasizes separation by size captures the mechanism most completely. The gel’s composition and concentration also affect how well different sizes resolve, but the key idea is the combination of electric-driven migration and the gel’s pore-based sieving.