DNA gels are used for visualising DNA using electrophoresis.
DNA gel electrophoresis is a method for separating different lengths of DNA on the basis of size. A solution of mixed-length DNA is pipetted into small wells at the ‘top’ of the gel which is formed of a compound called agarose. When in use, the gel is submerged in a buffer such as Tris-Acetate-EDTA (TAE). Because DNA is negatively charged, it migrates towards the positive electrode when a current is applied across the gel. The DNA migrates through the gel at different rates, dependant on the length and weight of the DNA strand.
Once the gel has been stopped, the DNA strands are visualised by UV-light. A dye, such as ethidium bromide, which stacks between the bases of DNA, fluoresces to show where the bands have migrated to. Small fragments of DNA will have migrated further than long fragments of DNA.