Why do bands appear on a gel after restriction digestion?

Correct answer: DNA fragments separate according to size

The governing concept is agarose-gel electrophoresis. DNA molecules carry an overall negative charge because of their phosphate groups, so an electric field moves them through the gel toward the positive electrode. Smaller fragments pass through the gel pores more easily and migrate farther than larger fragments. After staining, many molecules of similar length accumulate in visible bands. Thus option A is correct: the band pattern represents the sizes of fragments produced by restriction digestion. The enzyme does not turn into protein, the buffer does not solidify to create bands, and water cannot become DNA.