Question

The enzyme which adds phosphate group to the free 5' terminus of a DNA sequence is

• A. adenosine kinase
• B. alkaline phosphatase
• C. polynucleotide kinase
• D. terminal deoxynucleotidyl transferase

Hint:

PNK = adds 5' phosphate; AP = removes phosphate; TdT = adds nucleotides to 3' end.

Answer 1

The Correct Option is C

Polynucleotide kinase (PNK) is an important enzyme in DNA repair and molecular cloning.
It specifically catalyzes the transfer of a phosphate group from ATP to the 5' hydroxyl terminus of DNA or RNA.
This phosphorylation is essential for ligation reactions, because DNA ligase requires a 5' phosphate to join nucleic acids.
Adenosine kinase phosphorylates adenosine, not DNA terminus.
Alkaline phosphatase removes phosphate groups instead of adding them, which is the opposite reaction.
Terminal deoxynucleotidyl transferase adds nucleotides to the 3' end, not phosphate to the 5' end.
Therefore, the correct enzyme that adds phosphate to a free 5' end is polynucleotide kinase.