What is the role of DNA polymerase?

• A. Enzyme that unwinds the DNA double helix
• B. Enzyme that ligates DNA fragments
• C. Enzyme that synthesizes new DNA strands; reads the original strand and adds the complimentary nucleotide; requires RNA primer
• D. Enzyme that transcribes DNA into RNA

Answer: (C)

DNA polymerase builds new DNA by reading the template strand and adding complementary nucleotides to form a new strand. It can only start from an existing 3' end, so an RNA primer laid down by primase is needed to begin synthesis. The enzyme copies the template in the 3' to 5' direction while synthesizing the new strand in the 5' to 3' direction, and it checks its work with proofreading to correct mistakes. This role is distinct from helicase, which unwinds the DNA double helix, from ligase, which joins fragments, and from RNA polymerase, which transcribes DNA into RNA.