Question

Ethidium bromide is a stain that binds specifically to nucleic acids (DNA or RNA). If viewed under ultraviolet light, it gives ________ color to the nucleic acid bands.

• A. yellow
• B. orange
• C. blue
• D. green

Answer 1

The correct option is B orange

Ethidium bromide is a molecule that acts as an intercalating agent for nucleic acids. It does so by getting itself inserted in the spaces between the base pairs present in double helical DNA. This absorbs wavelength corresponding to ultraviolet radiation (UV absorbance maxima is 300 and 360nm) and emits another wavelength that is visible to us. This corresponds to the orange color (emission maxima is at 590nm). Hence, the bands of DNA obtained after gel electrophoresis can be visualised using ethidium bromide stain (added to the gel while preparation) followed by their exposure to ultraviolet light.

Fig: (a) Ethidium bromide (EtBr) as an intercalating agent
(b) Gel containing DNA bands after ethidium bromide staining and UV exposure