Gel electrophoresis is used to separate DNA fragments by size.

• A. Gel electrophoresis is used to separate DNA fragments by size
• B. It measures gene expression
• C. It sequences whole genomes
• D. It amplifies DNA segments

Answer: (A)

DNA fragments separate by size in gel electrophoresis because DNA carries a uniform negative charge along its backbone, so when an electric field is applied, the fragments migrate toward the positive electrode. The gel matrix acts like a sieve: smaller fragments pass through the pores more easily and move faster than larger ones, yielding distinct bands that reflect fragment lengths. This technique is commonly used to estimate fragment sizes after methods like restriction enzyme digestion or PCR, using a DNA ladder for reference. The other options describe different molecular biology techniques—measuring gene expression, sequencing whole genomes, or amplifying DNA segments—none of which are the primary purpose of gel electrophoresis itself.