Q.9 During DNA repair, which of the following enzymes is involved in phosphodiester bond formation ?
1. DNA polymerase
2. DNA ligase
3. Phosphodiesterase
4. Primase
DNA Ligase Seals Phosphodiester Bonds in DNA Repair
DNA ligase is the key enzyme that forms phosphodiester bonds during DNA repair by joining Okazaki fragments or repairing nicks in the backbone. This MCQ helps biology students master DNA repair mechanisms for exams like NEET or CSIR NET.
Question Breakdown
The question asks: “During DNA repair, which enzyme is involved in phosphodiester bond formation?” Options: 1. DNA polymerase, 2. DNA ligase, 3. Phosphodiesterase, 4. Primase.
Correct Answer
2. DNA ligase. It catalyzes the formation of phosphodiester bonds between a 3′-OH and a 5′-phosphate at nicks or breaks in DNA, using ATP to create a ligase-AMP intermediate before sealing the strand.
Option Explanations
DNA Polymerase (Option 1)
DNA polymerase synthesizes new DNA strands by adding nucleotides to the 3′-OH end, forming phosphodiester bonds during polymerization in replication and repair (e.g., base excision repair). However, it requires a primer and cannot join non-adjacent fragments or seal nicks without a template.
DNA Ligase (Option 2)
DNA ligase joins adjacent DNA fragments by forming phosphodiester bonds at single-strand breaks, essential in nucleotide excision repair, Okazaki fragment ligation, and recombination. Its mechanism involves three steps: AMP activation, transfer to 5′-phosphate, and 3′-OH attack to release AMP.
Phosphodiesterase (Option 3)
Phosphodiesterase hydrolyzes phosphodiester bonds, breaking down nucleic acids or second messengers like cAMP, rather than forming them. It acts in degradation pathways, not repair synthesis.
Primase (Option 4)
Primase synthesizes short RNA primers to initiate DNA synthesis by DNA polymerase, forming phosphodiester bonds in RNA but not in final DNA repair contexts. Primers are later removed and replaced.
Enzyme Functions Table
| Enzyme | Role in DNA Metabolism | Phosphodiester Bond Action |
|---|---|---|
| DNA Polymerase | Strand synthesis | Forms during elongation |
| DNA Ligase | Nick sealing | Forms to join fragments |
| Phosphodiesterase | Bond hydrolysis | Breaks bonds |
| Primase | Primer synthesis | Forms RNA primers |
DNA ligase ensures genome integrity by finalizing repair, distinguishing it from synthetic enzymes like polymerase.
