In electrophoresis, what primarily determines how fast a DNA fragment moves through the gel?

• A. The charge and size of the fragment
• B. The sequence of the fragment
• C. The color of the dye used
• D. The temperature

Answer: (A)

DNA movement in a gel is driven by the electric field pulling on the DNA’s negative backbone, but how fast a fragment travels is determined mainly by its size. The gel acts like a sieve: smaller fragments slip through pores more easily and experience less resistance, so they move farther in a given amount of time. Larger fragments face more friction as they collide with more pore walls, slowing their progress. The charge per length of DNA is roughly constant, so the primary difference in mobility comes from size rather than sequence. Visualization dye colors don’t affect movement, and while temperature can tweak gel viscosity slightly, it doesn’t set the mobility the way size does.