Tools of Biotechnology
Read the following passage carefully and answer the given questions.
It is evident that genetic engineering or recombinant DNA technology can be accomplished only if we have the key tools, i.e., restriction enzymes, polymerase enzymes, ligases, vectors and the host organism. DNA fragments are negatively charged molecules which can be separated by forcing them to move towards the anode under an electric field through a medium. Nowadays the most commonly used matrix is agarose which is a natural polymer extracted from sea weeds. The DNA fragments are resolved according to their size through sieving effect provided by the agarose gel. Hence, the smaller the fragment size, the farther it moves. If the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times. Such repeated amplification is achieved by the use of a thermostable DNA polymerase, which remain active during high temperature.
The separated DNA fragments can be visualized in the gel only after staining with:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ethidium bromide
DNA fragments are colourless, and pure DNA cannot be seen in visible light. After the gel run they have to be stained.
(1) Bromophenol blue — Incorrect. This is a tracking dye added to the DNA sample before the run. It shows how far the sample has moved. It does not stain the DNA fragments.
(2) Ethidium bromide — Correct. The gel is stained with ethidium bromide and then exposed to UV radiation. The DNA shows as bright orange bands.
(3) Crystal violet — Incorrect. Crystal violet is a stain used for bacteria. It is not used to stain DNA in an agarose gel.
(4) Silver chloride — Incorrect. This is not the stain used for DNA bands in an agarose gel.
Note 1: 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.
Note 2: A similar question was also asked in CUET (UG) 2026 on 20-05-2026 (Slot 2).
NCERT Extract:
Restriction Enzymes – …
