A DNA ladder, which is composed of varying lengths of DNA molecules, is used as a reference solution to estimate the size of unknown DNA fragments in gel electrophoresis. 

To visualize the DNA ladder, a radioactive or fluorescent dye is added to the DNA fragments before they are loaded onto the gel. The dye binds to the DNA, causing the fragments to fluoresce under UV light. The DNA ladder can be detected and visualized with a UV transilluminator or with an autoradiograph. The ladder appears as a series of distinct bands of varying sizes on the gel, with the smaller fragments migrating further than the larger ones.

The visible bands of the ladder serve as a reference for estimating the approximate size of the DNA fragments in the samples being analyzed.