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.
Elution is a crucial step in gel electrophoresis and defined as:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Separated bands of DNA are cut out from the agarose gel and extracted from the gel piece
In elution the DNA purified in this way is used to make recombinant DNA by joining it with cloning vectors.
(1) Cutting of DNA by the use of restriction endonucleases — Incorrect. This is restriction digestion. It comes before electrophoresis and makes the fragments that are separated.
(2) Separated bands of DNA are cut out from the agarose gel and extracted from the gel piece — Correct. This is the definition of elution.
(3) Staining method — Incorrect. Staining with ethidium bromide is done to see the bands. It is not elution.
(4) Introduction of DNA segment into a host — Incorrect. This is done later, with the recombinant DNA, and is not part of electrophoresis.
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 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.”