Separation and isolation of DNA fragments
Read the following passage carefully and answer the given questions.
The cutting of DNA by restriction endonucleases results in the fragments of DNA. These fragments can be separated by a technique known as gel electrophoresis. Since DNA fragments are negatively charged molecules, they can be separated by forcing them to move toward the electrode under an electric field through a medium. Nowadays, the most commonly used matrix is agarose, a natural polymer extracted from sea weeds. The DNA fragments separate through sieving effect provided by the agarose gel. The separated DNA fragments can be visualised only after staining the DNA with an appropriate dye followed by exposure to UV radiation. The separated bands of DNA are cut out from the agarose gel and extracted from the gel piece. The DNA fragments purified in this way are used in constructing recombinant DNA by joining them with cloning vectors.
In the gel electrophoresis, DNA fragments are visualised as :
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Bright orange colored band under exposure to UV light
Pure DNA fragments cannot be seen in visible light. The gel is first stained with ethidium bromide and then exposed to UV radiation. The DNA then shows as bright orange coloured bands.
(1) Bright orange colored band under exposure to fluorescent light — Incorrect. The colour is right, but the gel must be exposed to UV radiation, not to ordinary fluorescent light.
(2) Bright orange colored band under exposure to UV light — Correct. This is how the stained DNA shows up under UV (ultraviolet) light.
(3) Bright pink colored band under exposure to UV light — Incorrect. The exposure to UV (ultraviolet) light is right, but the bands are orange and not pink.
(4) Bright red colored band under exposure to sunlight — Incorrect. The bands are orange, and the DNA cannot be seen in visible light such as sunlight.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Restriction Enzymes – …
