Read the passage and answer the following question :
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 towards the anode under an electric field through a medium/matrix. The commonly used matrix is agarose which is a natural polymer extracted from sea weeds. The DNA fragments separate according to their size through sieving effect provided by the agarose gel. Hence, the smaller fragments move farther in the agarose gel. The separated DNA fragments can be visualised only after staining the DNA with ethidium bromide followed by exposure to UV radiation. Bright orange coloured bands of DNA can be observed. 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 cloning vectors.
What is elution ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Cutting of separated DNA fragments from agarose gel and extraction of DNA fragment.
Option (1) It is movement of negatively charged DNA fragments through agarose gel. — Incorrect. This describes gel electrophoresis itself. The negatively charged DNA fragments move through the agarose gel towards the anode and separate by size. Elution comes after this, when the separated bands are cut out.
Option (2) Extraction of DNA from the host. — Incorrect. This is isolation of DNA, the first step of the process.
Option (3) Extraction of DNA and treatment with restriction endonuclease. — Incorrect. This describes two earlier steps: isolation of DNA and its cutting by restriction endonucleases. Cutting makes the fragments that are then separated on the gel. Elution comes only after the fragments are separated.
Option (4) Cutting of separated DNA fragments from agarose gel and extraction of DNA fragment. — Correct. The passage says the separated bands are cut out of the gel and the DNA is extracted from the gel piece; this step is elution. The purified fragments are then joined with cloning vectors.
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.