DNA strands on a gel stained with ethidium bromide when viewed under UV radiation, appear as :

Ethidium bromide (EtBr) is a fluorescent dye that intercalates between the base pairs of double‑stranded DNA. When a gel containing DNA stained with EtBr is illuminated with ultraviolet (UV) light, the dye absorbs UV photons and re‑emits light at a longer wavelength.

Key points to understand the observed colour:

1. Absorption and emission spectra of EtBr:
• EtBr strongly absorbs light around 300 nm (UV region).
• After absorption, it fluoresces with a peak emission near 590 nm, which lies in the orange‑red part of the visible spectrum.

2. DNA’s role:
• The dye binds preferentially to DNA; therefore, regions of the gel that contain DNA become enriched with EtBr.
• The more DNA present, the stronger the fluorescence intensity in those zones.

3. Visual appearance under UV transillumination:
• The emitted orange‑red light from EtBr is seen as bright bands where DNA is located.
• Background gel without DNA emits little fluorescence, appearing dark.

Consequently, when the gel is viewed under UV illumination, the DNA‑containing lanes are seen as bright orange bands against a dark background.