Why must nucleotides be added in the 5' to 3' direction?

Why must nucleotides be added in the 5' to 3' direction?

• A. It ensures polymerase can read the template correctly.
• B. Because the 3' end contains the hydroxyl group necessary to form the new phosphodiester bond.
• C. Because the 5' end can't be extended due to lack of energy
• D. Because the template strand runs antiparallel

Answer: (B)

Nucleotides are added in the 5' to 3' direction because the chemistry of DNA synthesis requires a free 3' hydroxyl group to form the next phosphodiester bond. The 3' hydroxyl on the growing strand acts as a nucleophile, attacking the incoming nucleotide’s α-phosphate and linking the new nucleotide to the chain while releasing pyrophosphate. This chemical step can only occur at the 3' end, so synthesis must extend from 3' to 5' on the template (reading the template 3' to 5' and building the new strand 5' to 3'). The 5' end lacks a free hydroxyl to initiate the next bond, which is why extension cannot proceed from there.

Nucleotides are added in the 5' to 3' direction because the chemistry of DNA synthesis requires a free 3' hydroxyl group to form the next phosphodiester bond. The 3' hydroxyl on the growing strand acts as a nucleophile, attacking the incoming nucleotide’s α-phosphate and linking the new nucleotide to the chain while releasing pyrophosphate. This chemical step can only occur at the 3' end, so synthesis must extend from 3' to 5' on the template (reading the template 3' to 5' and building the new strand 5' to 3'). The 5' end lacks a free hydroxyl to initiate the next bond, which is why extension cannot proceed from there.