All of the following statements regarding DNA polymerase III in prokaryotes are correct EXCEPT:

Explanation

## DNA Polymerase III Structure and Function ### Core Catalytic Properties **Key Point:** DNA polymerase III is the main replicative enzyme in prokaryotes and has two distinct exonuclease activities with different roles. ### Exonuclease Activities of DNA Pol III | Activity | Direction | Function | Role | |----------|-----------|----------|------| | 3' → 5' exonuclease | Removes from 3' end | Proofreading | Removes mismatched nucleotides during synthesis | | 5' → 3' exonuclease | Removes from 5' end | NOT present in Pol III | This activity belongs to DNA Pol I | **High-Yield:** DNA Polymerase III lacks 5' to 3' exonuclease activity. This function is performed by **DNA Polymerase I**, which uses its 5' to 3' exonuclease activity to remove RNA primers left by primase and fill in the gaps with DNA nucleotides. ### Correct Properties of DNA Pol III 1. **3' to 5' exonuclease activity** — Present; enables proofreading and error correction 2. **Synthesis direction** — 5' to 3' (universal for all DNA polymerases) 3. **Primer requirement** — Absolutely requires an RNA primer; cannot initiate de novo 4. **5' to 3' exonuclease activity** — ABSENT in Pol III; present in Pol I **Clinical Pearl:** The distinction between Pol I and Pol III exonuclease activities is clinically relevant in understanding DNA repair mechanisms and why mutations in these enzymes cause genomic instability. **Mnemonic:** "Pol III is the **main** replicase (3' to 5' proofreading only); Pol I is the **cleanup** enzyme (5' to 3' primer removal)."