Gel electrophoresis
Read the following passage carefully and answer the given questions.
The cutting of DNA by molecular scissors results in 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 electrode under an electric field through a matrix. Nowadays, most commonly used matrix is agarose, which is a natural polymer. The DNA fragments resolve according to their size through sieving effect provided by agarose gel. Hence, smaller fragment size, farther it moves. The separated DNA fragments can be visualized only after staining DNA with a dye followed by exposure to radiation. The separated bands of DNA are cut out from agarose gel and extracted from gel piece. The DNA fragments purified in this way are used in construction recombinant DNA by joining them with cloning vectors.
Which one of the following enzymes is used to cut the DNA molecule?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Restriction endonuclease
Restriction enzymes are called molecular scissors. A restriction endonuclease recognises a specific sequence of bases in DNA and cuts both strands at that place. In this way it cuts a DNA molecule into fragments that can be separated by gel electrophoresis.
(1) Protease — Incorrect. It breaks down proteins. It is used to remove the proteins when DNA is isolated from the cell.
(2) DNA ligase — Incorrect. It does the opposite job. It joins cut DNA fragments, as when a gene is linked to a vector.
(3) Restriction endonuclease — Correct. It cuts the DNA at specific sites.
(4) Lysozyme — Incorrect. It breaks the cell wall of bacteria so that the DNA can be released. It does not cut DNA.
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:
Principles of Biotechnology – “… Stanley Cohen and Herbert Boyer accomplished this in 1972 by isolating the antibiotic resistance gene by cutting out a piece of DNA from a plasmid which was responsible for conferring antibiotic resistance. The cutting of DNA at specific locations became possible with the discovery of the so-called ‘molecular scissors’- restriction enzymes.”