Loading dyes are colored dyes that are mixed with a DNA sample before running electrophoresis. They serve three important functions when setting up gel electrophoresis:  

• Loading dyes color the sample, simplifying the loading process by providing a color indicator that makes it easier to visualize the sample. 
• They act as color indicators during gel electrophoresis, enabling researchers to estimate the distance that the nucleic acid fragments have migrated. 
• Loading dyes increase the density of the sample, ensuring that the DNA sinks into the gel and does not diffuse away into the buffer.