Which statement correctly describes the direction of DNA synthesis by DNA polymerase?

Which statement correctly describes the direction of DNA synthesis by DNA polymerase?

• A. DNA polymerase adds nucleotides to the 5' end of the growing strand, extending in 5' to 3' direction.
• B. DNA polymerase extends in the 3' to 5' direction.
• C. DNA polymerase adds nucleotides to the 3' end of the growing strand, extending in 5' to 3' direction.
• D. DNA polymerase can extend in either direction, depending on the template.

Answer: (C)

DNA synthesis by DNA polymerase occurs by adding nucleotides to the 3' end of the growing strand, so the new strand is built in the 5' to 3' direction. Each incoming nucleotide provides a phosphate that the enzyme uses to form a phosphodiester bond with the 3'-OH group of the last nucleotide. This chemistry requires a free 3' hydroxyl to attack the incoming nucleotide’s high-energy phosphate, which is why growth happens at the 3' end. As a result, the template is read in the 3' to 5' direction to synthesize the complementary strand in the 5' to 3' direction, and a primer with a free 3' OH is needed to start synthesis. Extending in the opposite direction would not be compatible with the enzyme’s catalytic chemistry.

DNA synthesis by DNA polymerase occurs by adding nucleotides to the 3' end of the growing strand, so the new strand is built in the 5' to 3' direction. Each incoming nucleotide provides a phosphate that the enzyme uses to form a phosphodiester bond with the 3'-OH group of the last nucleotide. This chemistry requires a free 3' hydroxyl to attack the incoming nucleotide’s high-energy phosphate, which is why growth happens at the 3' end. As a result, the template is read in the 3' to 5' direction to synthesize the complementary strand in the 5' to 3' direction, and a primer with a free 3' OH is needed to start synthesis. Extending in the opposite direction would not be compatible with the enzyme’s catalytic chemistry.