During nucleic acid polymerization, nucleoside triphosphates are added to which end of the backbone?

During nucleic acid polymerization, nucleoside triphosphates are added to which end of the backbone?

• A. Nucleoside diphosphates added to the 5' end
• B. Monophosphates added to the 2' end
• C. Nucleoside triphosphates added to the 3' end
• D. Free nucleosides added to the 3' end

Answer: (C)

The growing nucleic acid chain is extended at its 3' end. The 3' hydroxyl group of the last nucleotide acts as a nucleophile and attacks the α-phosphate of an incoming nucleoside triphosphate, forming a new phosphodiester bond and releasing pyrophosphate. This chemistry requires the incoming substrate to be a triphosphate, and the chain grows in the 5'→3' direction, with the 5' end carrying the terminal phosphate. Free nucleosides or monophosphates can’t be incorporated in this step, and the 2' position isn’t involved in the elongation chemistry. That’s why nucleoside triphosphates are added to the 3' end.

The growing nucleic acid chain is extended at its 3' end. The 3' hydroxyl group of the last nucleotide acts as a nucleophile and attacks the α-phosphate of an incoming nucleoside triphosphate, forming a new phosphodiester bond and releasing pyrophosphate. This chemistry requires the incoming substrate to be a triphosphate, and the chain grows in the 5'→3' direction, with the 5' end carrying the terminal phosphate. Free nucleosides or monophosphates can’t be incorporated in this step, and the 2' position isn’t involved in the elongation chemistry. That’s why nucleoside triphosphates are added to the 3' end.