What do we expose to UV rays(3rd para of image)?
Doesn't exposing DNA to UV rays can disrupt its structure?

Dear student, Nucleic acids (DNA and RNA) and proteins are colourless in a solution. Hence, these need to be stained so that we can observe the individual bands of DNA as produced by electrophoresis in th gel. Generally, ethidium bromide is used to stain DNA present in agarose gel. When this gel is exposed to UV light, the electrons in the ethidium molecule gets activated. They release energy in the form of light when returns to ground state. This makes the DNA bands visible to us which otherwise, will not be visible. DNA stained with ethidium bromide is not visible under natural light. Though UV rays cause mutation but the frequency of UV light used is low so it doesn't lead to any significant changes. Thanks and regards