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 at specific points in -
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Sugar-phosphate backbones
The passage says that once a restriction endonuclease finds its recognition sequence, it binds to the DNA and cuts each of the two strands of the double helix at specific points in their sugar-phosphate backbones. Each strand of DNA is a chain of nucleotides. The sugar of one nucleotide is joined to the phosphate of the next by a phosphodiester bond, forming the backbone. The nitrogenous bases stick out from this backbone and pair with the bases of the other strand. The enzyme breaks the phosphodiester bond in the backbone.
(1) Sugar-base backbones — Incorrect. The base is attached to the sugar as a side group. It does not form the backbone.
(2) Sugar-amino acid backbones — Incorrect. DNA has no amino acids. Amino acids are the units of proteins.
(3) Disulfide linkage — Incorrect. Disulfide bonds join parts of proteins (for example, the two chains of insulin). DNA has no disulfide links.
(4) Sugar-phosphate backbones — Correct. The cut is made in the sugar-phosphate backbone of each strand.
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.