Restriction endonuclease
Read the following passage carefully and answer the given questions.
Hind II always cuts DNA molecules at a particular point by recognizing a specific sequence of six base pairs. Besides Hind II, today we know more than 900 restriction enzymes that have been isolated from over 230 strains of bacteria, each of which recognizes different recognition sequence. The convention for naming these enzymes is the first letter of the name comes from the genus and the second two letters come from the species of the prokaryotic cell from which they were isolated. Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria. Restriction enzymes belong to a larger class of enzymes called nucleases. These are of two kinds: exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of DNA, whereas endonucleases make cuts at specific positions within the DNA. Each restriction endonuclease functions by ‘inspecting’ the length of a DNA sequence. Once it finds its specific recognition sequence, it will bind to the DNA and cuts each of the two strands of the double helix at specific points in their sugar-phosphate backbones. Each restriction endonuclease recognizes a specific palindromic nucleotide sequence in the DNA. These are groups of letters that form the same word when read both forward and backward, e.g., “MALAYALAM”.
Restriction endonucleases cut DNA -
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → At specific positions within DNA
The passage says that restriction enzymes belong to a larger class of enzymes called nucleases, which are of two kinds. Exonucleases remove nucleotides from the ends of the DNA. Endonucleases make cuts at specific positions within the DNA. A restriction endonuclease, as its name shows, is an endonuclease.
(1) At the ends of DNA — Incorrect. Removing nucleotides from the ends is the work of exonucleases.
(2) At specific positions within DNA — Correct. A restriction endonuclease cuts only at its own recognition sequence inside the DNA.
(3) At specific positions within RNA — Incorrect. Restriction endonucleases act on DNA, not RNA.
(4) At glycosidic linkage — Incorrect. The cut is made in the sugar-phosphate backbone, not at the bond between the sugar and the base.
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.