DNA polymerase synthesizes the complementary strand in which direction?

DNA polymerase synthesizes the complementary strand in which direction?

• A. 3' to 5'
• B. 5' to 3'
• C. Both directions
• D. None

Answer: (B)

DNA polymerase builds the new strand by adding nucleotides to the 3' end, so synthesis proceeds in the 5' to 3' direction. It reads the template strand in the opposite orientation, 3' to 5', to ensure correct base pairing. The chemical step requires a free 3' hydroxyl group on the growing strand to attack the incoming nucleotide's phosphate, forming a phosphodiester bond and releasing pyrophosphate. Because there is no free 3' OH at the 5' end to start extension, the enzyme cannot synthesize in the 3' to 5' direction. Even though strands are made continuously on one template and discontinuously on the other, every newly formed strand grows 5' to 3' relative to itself.

DNA polymerase builds the new strand by adding nucleotides to the 3' end, so synthesis proceeds in the 5' to 3' direction. It reads the template strand in the opposite orientation, 3' to 5', to ensure correct base pairing. The chemical step requires a free 3' hydroxyl group on the growing strand to attack the incoming nucleotide's phosphate, forming a phosphodiester bond and releasing pyrophosphate. Because there is no free 3' OH at the 5' end to start extension, the enzyme cannot synthesize in the 3' to 5' direction. Even though strands are made continuously on one template and discontinuously on the other, every newly formed strand grows 5' to 3' relative to itself.