What determines the rate at which DNA fragments move in gel electrophoresis?

• A. Base sequence.
• B. Color of the staining dye.
• C. Electric field orientation.
• D. Size of the fragments.

Answer: (D)

In gel electrophoresis, how fast a DNA fragment moves is determined mainly by its size. DNA has roughly the same charge per length, so the pulling force from the electric field is similar for fragments of different sizes. The gel acts as a sieve, and smaller fragments navigate the gel’s pores with less friction, so they travel faster toward the positive electrode. The color of the staining dye only marks the bands after the run and doesn’t affect movement; the electric field’s direction sets where the fragments go, but not the inherent rate for fragments of a given size; and base sequence doesn’t significantly change mobility for linear DNA, so size is the key factor.