Which of the following statements is correct regarding visualising DNA molecules separated by agarose gel electrophoresis?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Ethidium bromide stained DNA can be seen under exposure to UV light.
Pure DNA fragments cannot be seen in visible light, and they cannot be seen without staining. The gel is stained with ethidium bromide and then exposed to UV radiation; the DNA then shows up as bright orange coloured bands.
Option (1) DNA can be observed in visible light without staining. — Incorrect. Unstained DNA is invisible in ordinary light. Neither the stain nor UV light is used here.
Option (2) Ethidium bromide stained DNA can be observed in visible light. — Incorrect. Staining alone is not enough. Ethidium bromide stained DNA shows up only when the gel is exposed to UV light.
Option (3) DNA can be seen without staining under UV light. — Incorrect. UV light alone shows nothing. The DNA must first be stained with ethidium bromide.
Option (4) Ethidium bromide stained DNA can be seen under exposure to UV light. — Correct. Both steps are needed together: staining with ethidium bromide, followed by exposure to UV light, gives the orange bands.
NCERT Extract:
Restriction Enzymes – “… The separated DNA fragments can be visualised only after staining the DNA with a compound known as ethidium bromide followed by exposure to UV radiation (you cannot see pure DNA fragments in the visible light and without staining). You can see bright orange coloured bands of DNA in a ethidium bromide stained gel exposed to UV light (Figure 9.3).”