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.
Which one of the following dyes is used for the visualization of DNA in electrophoresis?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Ethidium bromide
The passage says that the separated DNA fragments can be seen only after they are stained and exposed to UV radiation. Pure DNA cannot be seen in visible light. The stain that NCERT names is ethidium bromide (EtBr). It gives bright orange bands of DNA under UV light.
(1) Acetocarmine — Incorrect. This is a stain used to colour chromosomes in cells under the microscope. It is not used for DNA in a gel.
(2) Bromophenol blue — Incorrect. It is a tracking dye that is loaded with the samples so that their movement can be followed. It does not stain DNA for viewing under UV light.
(3) Ethidium bromide — Correct. It stains the DNA, and the DNA then shows up under UV light.
(4) Silver chloride — Incorrect. It is not the DNA stain used in gel electrophoresis. The stain is ethidium bromide.
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:
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).”