Assertion (A): After agarose gel electrophoresis, separated DNA fragments need to be stained with ethidium bromide to be visualised.
Reason (R): Ethidium bromide binds to DNA molecules and fluoresces under UV light, making the DNA appear as bright orange coloured bands.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both A and R are true, and R is the correct explanation of A.
Assertion (A): After agarose gel electrophoresis, separated DNA fragments need to be stained with ethidium bromide to be visualised. — True. A is about seeing the DNA bands after gel electrophoresis. Pure DNA cannot be seen in visible light, so the separated fragments are visible only after staining with ethidium bromide and exposure to UV light.
Reason (R): Ethidium bromide binds to DNA molecules and fluoresces under UV light, making the DNA appear as bright orange coloured bands. — True. R is about what ethidium bromide does. The stain attaches to the DNA, and under UV light the stained DNA shows up as bright orange coloured bands.
Why R explains A? A says that the DNA must be stained before it can be seen. R gives the reason: the stain sits on the DNA and glows orange under UV light. Without the stain there would be nothing to glow, so the bands would stay invisible. So R is the correct explanation of A.
NCERT Extract:
Separation and Isolation of DNA Fragments – “… 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).”