In bacterial DNA replication, what is the primary function of DNA polymerase III?

• A. It joins Okazaki fragments by sealing nicks in the sugar-phosphate backbone (DNA ligase activity).
• B. It synthesizes new DNA strands by adding deoxyribonucleotides to the $3^{\prime }$ end of a primer, extending DNA in the $5\prime \to 3\prime$ direction.
• C. It unwinds the DNA double helix ahead of the replication fork (helicase activity).
• D. It synthesizes short RNA primers to initiate DNA synthesis (primase activity).

Answer: (B)

Verified step by step guidance

1. Understand the role of DNA polymerase III in bacterial DNA replication by identifying its main enzymatic activity.
2. Recall that DNA polymerase III is responsible for synthesizing new DNA strands by adding nucleotides to an existing primer.
3. Note that DNA polymerase III adds deoxyribonucleotides specifically to the 3\' end of the primer, which means DNA synthesis occurs in the 5\' to 3\' direction.
4. Distinguish DNA polymerase III's function from other enzymes: DNA ligase joins Okazaki fragments, helicase unwinds the DNA helix, and primase synthesizes RNA primers.
5. Summarize that the primary function of DNA polymerase III is to extend the DNA strand by adding nucleotides to the 3\' end of the primer, synthesizing DNA in the 5\' to 3\' direction.