The enzyme which adds phosphate group to the free 5' terminus of a DNA
sequence is
Options:
(A) adenosine kinase
(B) alkaline phosphatase
(C) polynucleotide kinase
(D) terminal deoxynucleotidyl transferase
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2021 Q.25)**
**1. Governing Concept**:
- **Topic**: Molecular Biology & Nucleic Acid Enzymes
- **T4 Polynucleotide Kinase (PNK)** transfers a $\gamma$-phosphate group from ATP to the free $5'\text{-OH}$ terminus of single-stranded or double-stranded DNA or RNA molecules, widely used for $5'$ end-labeling with $^{32}\text{P}$ or preparing oligonucleotides for ligation.
**2. Option Analysis**:
- **Option (A) [adenosine kinase] is INCORRECT**: Phosphorylates adenosine to AMP in purine salvage.
- **Option (B) [alkaline phosphatase] is INCORRECT**: Removes $5'$ phosphate groups (dephosphorylation).
- **Option (C) [polynucleotide kinase] is CORRECT**: Catalyzes phosphorylation of $5'$ termini of polynucleotide chains.
- **Option (D) [terminal deoxynucleotidyl transferase] is INCORRECT**: Adds template-independent dNTPs to $3'$ ends.
**3. Final Answer**:
- **Correct Choice**: **C**
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