Endonuclease are enzyme responsible for the cleavage of:

Question

Accepted Answer

The correct option is 1: Phospho-di-ester bonds

Endonucleases are a class of enzymes belonging to the larger group known as nucleases, and their fundamental role is to manipulate the structure of nucleic acids (DNA and RNA). The backbone of every DNA and RNA strand is constructed from a repeating sequence of sugar and phosphate molecules, which are held together by strong covalent bonds called **phosphodiester bonds**. An endonuclease is specifically designed to catalyze the hydrolysis (cleavage) of these particular covalent bonds *within* the interior of a polynucleotide chain, creating fragments of the original nucleic acid molecule. The precise and sequence-specific cleavage of these bonds by enzymes like restriction endonucleases is the cornerstone of genetic engineering and recombinant DNA technology.

The incorrect option is 2: Hydrogen bonds

Hydrogen bonds are weak, non-covalent, electrostatic attractions that form between the complementary nitrogenous bases (Adenine-Thymine and Guanine-Cytosine) to hold the two strands of the DNA double helix together. They are essential for the structural integrity of the double helix, but they are not the target of endonuclease enzymes. The process of breaking these bonds is called denaturation or melting, and it is primarily achieved through high temperatures, as seen in the first step of Polymerase Chain Reaction (PCR), or by specialized unwinding enzymes known as *helicases*.

The incorrect option is 3: N-glycosidic linkage

The N-glycosidic linkage is the covalent bond that connects the nitrogenous base (Adenine, Guanine, Cytosine, Thymine, or Uracil) to the pentose sugar molecule (deoxyribose or ribose) within a single nucleotide. This bond is not cleaved by endonucleases, as its removal would not break the nucleic acid backbone. Instead, this specific bond is the target of *DNA glycosylases*, which are repair enzymes involved in the Base Excision Repair pathway. These enzymes detect and excise a damaged or incorrect base by hydrolyzing the N-glycosidic linkage, leaving behind an abasic site which is then repaired by other enzymes.

The incorrect option is 4: Peptide bonds

Peptide bonds are the strong covalent bonds responsible for linking individual amino acid monomers together to form a polypeptide chain, which is the primary structure of a protein. The enzymes that are responsible for cleaving (breaking down) these bonds are known as *proteases* or *peptidases*. Endonucleases are a class of enzymes that act exclusively on nucleic acids (DNA and RNA), whereas proteases act exclusively on proteins. Therefore, endonucleases have no catalytic role in the breakdown or cleavage of peptide bonds.