Question 22:
Enzymes that create a phosphodiester bond between the 3’ end of one DNA segment and the 5’ end of another are called:
DNA ligases join DNA fragments by forming phosphodiester bonds between 3′ OH and 5′ phosphate ends. The correct enzyme class is option (B).
Correct Answer
(B) Ligase
Ligases catalyze phosphodiester bond formation in replication, repair, and recombinant DNA tech, sealing nicks via ATP/NAD+ dependent mechanisms.
Key Concept
The reaction proceeds in three steps: (1) enzyme-AMP formation, (2) AMP transfer to 5′ phosphate creating DNA-adenylate, (3) 3′ OH nucleophilic attack seals the bond, releasing AMP.
Option Analysis
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(A) Lyase: Incorrect; cleaves bonds by elimination (e.g., aldolase), forming double bonds without hydrolysis.
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(B) Ligase: Correct; joins molecules using energy from ATP/NADPH, essential for Okazaki fragment joining.
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(C) Oxidoreductase: Incorrect; transfers electrons/hydrogen in redox reactions (e.g., dehydrogenases).
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(D) Hydrolase: Incorrect; cleaves bonds using water (e.g., nucleases), opposite of ligation.
| Option | Enzyme Class | Function Example | Status |
|---|---|---|---|
| A | Lyase | C-C bond cleavage | Wrong |
| B | Ligase | Phosphodiester join | Correct |
| C | Oxidoreductase | Electron transfer | Wrong |
| D | Hydrolase | Hydrolytic cleavage | Wrong |
Exam Preparation Tips
Recall EC classification: ligases = EC 6. T4 DNA ligase (viral) and E. coli ligase (NAD+) vs. eukaryotic (ATP). Key for NEET molecular biology.
