Read the following passage carefully and answer the given questions.
The separated DNA fragments can be visualized only after staining the DNA with ethidium bromide followed by exposure to UV radiation. Bright orange colored bands of DNA in ethidium bromide stained gel exposed to UV light can be seen. The separated bands of DNA are cut out from the agarose gel and extracted from the gel piece. This step is known as elution. The DNA fragments purified in this way are used in constructing recombinant DNA by joining them with a cloning vector.
The DNA band appears orange due to -
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Staining with ethidium bromide and exposed to UV light
Option (1) Staining with acetocarmine and exposed to UV light — Incorrect. Acetocarmine is a red stain used to colour chromosomes for viewing under a light microscope. It is not used to see DNA bands in a gel.
Option (2) Staining with ethidium bromide and exposed to UV light — Correct. Pure DNA cannot be seen in visible light without a stain. Ethidium bromide binds to DNA. Under ultraviolet (UV) light it glows, and the DNA shows up as bright orange bands.
Option (3) Staining with ethidium bromide and exposed to infra red light — Incorrect. The stain is right, but the light is wrong. Infra-red light is heat radiation. It does not make ethidium bromide glow, so the bands stay invisible.
Option (4) Staining with acetocarmine and exposed to infra-red light — Incorrect. Both the stain and the light are wrong. Acetocarmine is a chromosome stain, and infra-red light makes no DNA band glow.
Note: This passage is taken from NCERT as it is. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.