Question

In gel electrophoresis, DNA fragments move across a gel. What causes them to move?

Answer 1

Electric field. From negative to positive.

Answer 2

The negatively charged phosphate in DNA enables it to migrates along the gel from negative to positive. And the other factor is the length of your DNA fragment. Longer fragment will stuck on the upper part of the gel. On the contrary, shorter fragment will easily migrates to the lower part.