Which of the following enzyme is known as the molecular scissors ?

Question

Accepted Answer

The correct option is 3: Restriction endonuclease

Restriction endonucleases are the cornerstone of molecular biology and genetic engineering, earning the descriptive nickname "molecular scissors" due to their precise ability to cut DNA. These enzymes are naturally found in bacteria, where they serve as a defense mechanism against invading viruses (bacteriophages) by cleaving the viral DNA at specific locations, a process called restriction. What makes them so powerful as a tool is their highly specific nature: each restriction enzyme recognizes a unique, short, and palindromic sequence of DNA, known as the recognition site. Once the recognition site is located, the enzyme cleaves the phosphodiester bonds on both strands of the DNA double helix. This cutting action can result in two types of ends: "blunt ends," where the cut is straight through the molecule, or more often, "sticky ends," which are staggered cuts that leave short, single-stranded overhangs. These sticky ends are crucial for recombinant DNA technology because they can easily base-pair with complementary sticky ends from a different DNA fragment cut with the same enzyme, enabling the creation of a chimeric or recombinant DNA molecule. Without the precise, site-specific cutting capability of restriction endonucleases, the entire field of gene manipulation would not exist.

The incorrect option is 1: Lyase

Lyases are a class of enzymes that catalyze the cleavage of chemical bonds through reactions other than hydrolysis and oxidation. Their main function involves adding a group to a double bond or removing a group to form a double bond. For instance, they are vital in metabolic pathways like glycolysis, where aldolase cleaves fructose-1,6-bisphosphate. While they break bonds, their action is generally associated with the removal of small molecules like H2O, NH3, or CO2 to form double bonds, and not the targeted cleavage of the DNA backbone that defines the "molecular scissors." Therefore, the general bond-cleaving action of a lyase is distinct from the sequence-specific DNA cutting performed by a restriction endonuclease.

The incorrect option is 2: Ligase

DNA ligase is often referred to as the "molecular glue" because its primary function is the exact opposite of the molecular scissors. This enzyme catalyzes the formation of a phosphodiester bond between the 3'-hydroxyl end of one nucleotide and the 5'-phosphate end of another. In the context of genetic engineering, after the restriction endonuclease has cut DNA fragments and complementary sticky ends have annealed, DNA ligase is used to permanently weld these fragments together. It seals the nicks in the sugar-phosphate backbone, creating a stable, continuous, recombinant DNA molecule. Therefore, it is a crucial partner in the process, but its function is joining, not cutting.

The incorrect option is 4: Opines

Opines are not enzymes, but rather they are a class of low molecular weight organic compounds, specifically derivatives of amino acids and sugars. These compounds are typically produced in plants that have been genetically transformed by the bacterium *Agrobacterium tumefaciens*, which causes crown gall tumors. The bacteria integrate their Ti plasmid DNA into the plant's genome, forcing the plant to synthesize opines, which then serve as a unique carbon and nitrogen source for the bacterium. Since opines are metabolic substrates and not catalytic proteins, they cannot be classified as enzymes, and certainly not as the molecular scissors.