What is gel electrophoresis used for in molecular biology?

• A. To amplify DNA sequences.
• B. To separate DNA fragments by size using an electric field; results visualized with a stain.
• C. To sequence DNA.
• D. To ligate fragments together.

Answer: (B)

Gel electrophoresis is used to separate DNA fragments by size in a gel matrix while an electric field is applied. DNA is negatively charged, so it moves toward the positive electrode; the gel acts like a sieve, and smaller fragments migrate faster than larger ones. The pattern of bands you see corresponds to fragment sizes, which can be estimated by comparison with a DNA ladder and seen by staining the gel with a dye (such as ethidium bromide or a safer alternative) and visualizing under UV or blue light. This technique is commonly used to check PCR products, analyze restriction enzyme digestion results, and verify cloning fragments. It does not amplify DNA, determine sequence, or join fragments, which are done by other methods.