Practice question

Ethidium bromide is a fluorescent intercalator whose photophysics change dramatically upon DNA binding. Free EtBr in aqueous solution emits weakly, but when intercalated between base pairs, hydrophobic environment shields it and fluorescence quantum yield increases about 20-fold. Upon excitation with UV around 302 nm or 365 nm, the DNA-EtBr complex emits orange-red fluorescence with emission maximum near 590 nm. This property enables visualization of DNA bands in agarose gels under UV transilluminator. Shorter wavelengths like 260 nm or 400 nm are absorption peaks, not emission maxima.