Final answer:
In gel electrophoresis, the different bands are formed because DNA molecules have different sizes, with smaller fragments moving faster through the gel matrix, thus separating according to length.
Step-by-step explanation:
In gel electrophoresis of DNA, the different bands in the final gel form primarily because the DNA molecules have different sizes. DNA molecules possess a net negative charge due to the phosphate groups in their backbone, which causes them to migrate towards the positive electrode when an electric field is applied across the gel. However, the separation is based on size, with smaller DNA fragments moving faster and further through the gel's porous matrix, while larger fragments move more slowly and are retained closer to the well where they were initially loaded. This process results in the formation of distinct bands, each representing DNA fragments of a particular length. The nucleic acids in the gel are not visible until they are stained with a compound that allows them to be seen, like a DNA-specific dye. When observed post-staining, the bands corresponding to different-sized DNA fragments become visible, enabling analysis.