In gel electrophoresis, how does fragment size affect migration distance?

• A. Larger fragments move faster and farther
• B. Fragment size has no effect on movement
• C. Only extremely small fragments move
• D. Smaller fragments move faster and farther

Answer: (D)

In gel electrophoresis, DNA fragments are pulled through a gel matrix by an electric field, and all fragments carry the same kind of negative charge along their backbone. The gel acts like a sieve: smaller fragments thread through the pores with less resistance, so they move through the gel more quickly and travel farther in the same amount of time. Larger fragments face more friction as they navigate the dense network, so they migrate more slowly and end up closer to where they started. Because of this size-dependent hindrance, smaller fragments move faster and farther than larger ones. This is why gel electrophoresis separates DNA by fragment size.