GATE 2021 BT – Question 25
The enzyme adding phosphate to the free 5′ terminus of DNA is
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Correct answer: (C) polynucleotide kinase
Explanation
Match the enzyme with the reaction it catalyses.
- **Polynucleotide kinase (PNK):** transfers the $\gamma$-phosphate of ATP to the free **5′-hydroxyl** end of DNA or RNA. It is used to label DNA ends with $^{32}P$ and to phosphorylate inserts before ligation. ✓
- **Alkaline phosphatase:** does the opposite, removing 5′ phosphates (so a vector cannot re-close on itself).
- **Terminal deoxynucleotidyl transferase:** adds nucleotides to the 3′ end without a template.
- **Adenosine kinase:** phosphorylates the free nucleoside adenosine, not DNA.
Answer **polynucleotide kinase** (C).