Which technique separates DNA fragments by size using an electric field?

• A. Cloning
• B. Gel Electrophoresis
• C. Sequencing
• D. Transcription

Answer: (B)

DNA fragments can be separated by size when an electric field is applied through a gel matrix. DNA carries a negative charge from its phosphate backbone, so in an electric field it migrates toward the positive electrode. The gel acts like a sieve: smaller fragments navigate the pores more easily and move farther in a given time, while larger fragments are slowed down. This creates distinct bands, each representing DNA fragments of similar sizes. A ladder with known fragment sizes helps you estimate how big the sample fragments are, and staining makes the bands visible under light. This technique is commonly used to check digestion results, verify PCR product sizes, or prepare samples for cloning or sequencing. The other options perform different tasks: cloning makes copies of DNA, sequencing reveals the exact nucleotide order, and transcription produces RNA from DNA.