Which enzyme adds nucleotides to a growing DNA strand and works in the 5' to 3' direction?

Which enzyme adds nucleotides to a growing DNA strand and works in the 5' to 3' direction?

• A. DNA polymerase
• B. Primase
• C. DNA ligase
• D. Helicase

Answer: (A)

DNA polymerase is the enzyme that builds a new DNA strand by adding nucleotides to the 3' end of the growing chain, and it always works in the 5' to 3' direction. It uses the template strand to guide which nucleotide to add and forms the phosphodiester bonds that link nucleotides together, extending the chain from a pre-existing primer. The reason synthesis goes 5' to 3' is because the incoming nucleotide carries a 5' phosphate and a 3' hydroxyl on the growing strand; the polymerase catalyzes bond formation with the 3' end, enabling continuous addition. Primase, on the other hand, makes the RNA primer that provides the starting point for DNA synthesis but does not continue DNA synthesis itself. Helicase unwinds the double helix to separate strands, and ligase seals nicks between fragments after synthesis is complete.

DNA polymerase is the enzyme that builds a new DNA strand by adding nucleotides to the 3' end of the growing chain, and it always works in the 5' to 3' direction. It uses the template strand to guide which nucleotide to add and forms the phosphodiester bonds that link nucleotides together, extending the chain from a pre-existing primer. The reason synthesis goes 5' to 3' is because the incoming nucleotide carries a 5' phosphate and a 3' hydroxyl on the growing strand; the polymerase catalyzes bond formation with the 3' end, enabling continuous addition.

Primase, on the other hand, makes the RNA primer that provides the starting point for DNA synthesis but does not continue DNA synthesis itself. Helicase unwinds the double helix to separate strands, and ligase seals nicks between fragments after synthesis is complete.