Comprehension:
Restriction Enzymes
Read the following passage carefully and answer the given questions.
Genetic engineering is the combination of techniques to alter the chemistry of genetic material (DNA and RNA), to introduce these into host organisms and thus change the phenotype of the host organism. The first restriction endonuclease–Hind II, whose functioning depended on a specific DNA nucleotide sequence was isolated and characterised five years later. It was found that Hind II always cut DNA molecules at a particular point by recognising a specific sequence of six base pairs. This specific base sequence is known as the recognition sequence for Hind II. Besides Hind II, today we know more than 900 restriction enzymes that have been isolated from over 230 strains of bacteria each of which recognise different recognition sequences. Restriction enzymes belong to a larger class of enzymes called nucleases namely exonucleases and endonucleases. The DNA fragments can be separated according to their size through sieving effect by gel electrophoresis. Hence, the smaller the fragment size, the farther it moves.
'Molecular scissors' term is used for:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Restriction endonuclease
(1) Reverse transcriptase — Incorrect. This enzyme makes a DNA copy from an RNA template. It does not cut DNA.
(2) DNA ligase — Incorrect. This enzyme joins pieces of DNA together. It is used after the cutting is done, so it is the glue, not the scissors.
(3) Polymerase — Incorrect. A polymerase builds a new strand of nucleotides. It makes DNA and does not cut it.
(4) Restriction endonuclease — Correct. A restriction endonuclease cuts DNA at a specific place. It first finds its particular recognition sequence and then cuts both strands there. Because it cuts DNA at set positions, it is called a molecular scissors.
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.”