Question

DNA ligase is the molecular glue or gum, which joins together the cut-ends of DNA by creating

• A. Phosphotriester bond
• B. Phosphodiester bond
• C. Hydrogen Bond
• D. N-glycosidic bond

Hint:

DNA ligase acts as a molecular glue by catalyzing the formation of a covalent phosphodiester bond between adjacent 3'-OH and 5'-\(\text{PO}_4^{3-}\) groups, sealing nicks in the sugar-phosphate backbone.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
DNA ligase is an essential enzyme used in DNA replication, repair, and recombinant DNA technology to join DNA fragments.
It functions by restoring the structural integrity of the double helix's sugar-phosphate backbone.

Step 2: Detailed Explanation:
DNA ligase catalyzes the sealing of nicks in double-stranded DNA.
This reaction requires an energy source, which is typically ATP in eukaryotes and bacteriophages, or \(\text{NAD}^+\) in certain bacteria.
The enzyme activates the 5'-phosphate group at the nick by transferring an AMP group to it, creating an activated pyrophosphate bond.
The 3'-hydroxyl group of the adjacent nucleotide then performs a nucleophilic attack on this activated 5'-phosphate, releasing the AMP group.
This reaction forms a covalent phosphodiester bond that bridges the nick in the sugar-phosphate backbone.
Phosphotriester bonds (A) do not occur naturally in DNA.
Hydrogen bonds (C) are non-covalent interactions that stabilize base-pairing between complementary strands rather than forming the backbone.
N-glycosidic bonds (D) link the nitrogenous bases to the deoxyribose sugars, a reaction catalyzed by glycosylases rather than ligases.

Step 3: Final Answer:
Therefore, DNA ligase joins DNA fragments by creating a phosphodiester bond, which corresponds to option (B).