Adds new DNA nucleotides to a replicating DNA molecule.

• A. DNA polymerase III
• B. Helicase
• C. Telomere
• D. Amino acid

Answer: (A)

During DNA replication, new nucleotides are added to the growing DNA chain by enzymes that catalyze the formation of phosphodiester bonds in the 5' to 3' direction. The primary enzyme in bacteria responsible for this synthesis is DNA polymerase III, which carries out most of the nucleotide addition on both the leading and lagging strands. It works with a sliding clamp to remain highly processive, extending the chain by incorporating correct dNTPs that pair with the template. The addition starts from an RNA primer and proceeds as the fork opens, with polymerase III extending from the primer’s 3' end. Other players include helicase, which unwinds the double helix, and primase, which provides the primers; after synthesis, polymerase I replaces RNA primers with DNA. Telomeres are simply chromosome end sequences, not enzymes, and amino acids are the building blocks of proteins, not nucleotides.