Read the passage and answer the following question :
In 1963, enzymes that restricted the growth of bacteriophage in Escherichia coli were discovered. Among them, restriction endonucleases played a vital role. Five years later, Hind II, the first restriction endonuclease, was isolated and characterized. Hind II was known for its ability to cut DNA molecules at a specific point by recognizing a unique six-base pair sequence, which was termed the "recognition sequence" for Hind II. Currently, over 900 restriction enzymes have been identified from more than 230 bacterial strains, each recognizing different recognition sequences. For example, EcoRI is derived from Escherichia coli RY 13, where the letter 'R' represents the name of the strain, and the Roman numeral indicates the order of enzyme isolation from that strain. Restriction enzymes belong to a larger class of enzymes that can be divided into two types: exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of DNA, while endonucleases make cuts at specific positions within the DNA. Each restriction endonuclease "inspects" the length of a DNA sequence until it finds its specific recognition sequence. Once located, it binds to the DNA and cuts both strands of the double helix at specific points in their sugar-phosphate backbones. Notably, each restriction endonuclease recognizes a specific palindromic nucleotide sequence in the DNA.
Which of the following enzymes belongs to the class of enzymes called nucleases?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Restriction enzymes
The passage states that “Restriction enzymes belong to a larger class of enzymes that can be divided into two types: exonucleases and endonucleases.” Both types are nucleases, enzymes that act on the phosphodiester bonds of nucleic acids. So restriction enzymes belong to the class of nucleases. Exonucleases remove nucleotides from the ends of DNA, and endonucleases, such as restriction endonucleases, cut within the DNA.
Option (1) Ligase enzymes — Incorrect. DNA ligase does not break DNA. It acts on cut DNA molecules and joins their ends, so it is not a nuclease.
Option (2) Polymerase enzymes — Incorrect. DNA polymerase builds new DNA strands by adding nucleotides on a template. It does not cut DNA.
Option (3) Restriction enzymes — Correct. Restriction enzymes are nucleases. The ones used in genetic engineering are endonucleases that cut DNA at specific recognition sequences.