Which of the following is true about DNA polymerase I?

Explanation

## Correct Answer: D. It is found in prokaryotes DNA polymerase I (Pol I) is a prokaryotic enzyme with a molecular weight of ~103 kDa, discovered by Arthur Kornberg in *E. coli*. The defining characteristic of Pol I is its **exclusive presence in prokaryotes**—it is entirely absent in eukaryotic cells, which instead rely on DNA polymerases α, δ, and ε. In prokaryotes, Pol I plays a critical "cleanup" role: it removes RNA primers laid down by primase (which synthesizes the primers, not Pol I itself) and fills in the resulting gaps with DNA nucleotides. This 5′→3′ exonuclease activity combined with polymerase activity makes Pol I essential for completing DNA replication, particularly on the lagging strand where multiple Okazaki fragments must be joined. However, Pol I is NOT the primary replicative enzyme—that role belongs to **DNA polymerase III** in prokaryotes, which synthesizes the bulk of both leading and lagging strands. The distinction between Pol I and Pol III is a classic NEET PG discriminator: Pol III does the heavy lifting (high processivity, ~500,000 nucleotides per binding), while Pol I performs the finishing work (low processivity, ~20–100 nucleotides per binding). This prokaryote-specific architecture has no direct eukaryotic parallel, making the cellular localization the most reliable identifier of Pol I. ## Why the other options are wrong **A. It synthesizes RNA primers** — This is wrong because **primase** (a specialized RNA polymerase), not Pol I, synthesizes RNA primers. Pol I's role is to *remove* these primers using its 5′→3′ exonuclease activity and then fill the gaps with DNA. Confusing primer synthesis with primer removal is a common NBE trap that exploits incomplete understanding of the division of labor in replication. **B. It is the primary enzyme of DNA synthesis** — This is wrong because **DNA polymerase III** is the primary replicative enzyme in prokaryotes, responsible for ~99% of chromosomal DNA synthesis. Pol I is a secondary, 'finishing' enzyme with much lower processivity. Students often confuse Pol I's historical importance (first polymerase discovered) with functional importance in replication. **C. It is involved in the creation of Okazaki fragments** — This is wrong because Okazaki fragments are *synthesized* by Pol III on the lagging strand. Pol I is involved in *joining* these fragments by removing primers and filling gaps, not in their creation. This option exploits the misconception that Pol I's role in lagging strand completion means it creates the fragments themselves. ## High-Yield Facts - **DNA Pol I** is found exclusively in **prokaryotes** (*E. coli*, *Bacillus*, etc.); eukaryotes have Pol α, δ, ε instead. - **DNA Pol III** (not Pol I) is the primary replicative enzyme in prokaryotes with high processivity (~500,000 nt/binding). - **Primase** (not Pol I) synthesizes RNA primers; Pol I removes them via 5′→3′ exonuclease and fills gaps with DNA. - **Pol I processivity** is ~20–100 nucleotides per binding event, making it suitable for gap-filling, not bulk synthesis. - **Okazaki fragments** are synthesized by Pol III; Pol I joins them by removing primers and filling the resulting nicks. ## Mnemonics **Pol I vs Pol III in Prokaryotes** **Pol III = Main (3 = primary), Pol I = Finishing (1 = last step)**. Pol III does bulk synthesis; Pol I removes primers and fills gaps. Use when deciding which polymerase does what in bacterial replication. **The 'Cleanup Crew' Rule** Pol I is the cleanup crew: it removes RNA primers (5′→3′ exonuclease) and fills the gaps. Primase lays primers; Pol I removes them. Remember: **Primase makes, Pol I takes (away).** ## NBE Trap NBE pairs Pol I with "primary synthesis" or "Okazaki fragment creation" to exploit the historical prominence of Pol I (first polymerase discovered) and students' confusion about which enzyme does what on the lagging strand. The correct answer—"found in prokaryotes"—is the safest discriminator because it is unambiguous and requires no functional confusion. ## Clinical Pearl In clinical microbiology labs in India, understanding Pol I is relevant when studying bacterial antibiotic resistance and DNA repair mechanisms. Fluoroquinolones (e.g., ciprofloxacin, commonly used in Indian hospitals) target bacterial DNA gyrase, not Pol I, but knowledge of prokaryotic replication machinery helps explain why certain mutations confer resistance. Additionally, in molecular diagnostics (PCR, DNA sequencing), recognizing that Pol I is prokaryote-specific helps distinguish bacterial contamination from eukaryotic template in clinical samples. _Reference: Lehninger Principles of Biochemistry (Ch. 25: DNA Replication); Molecular Biology of the Gene by Watson et al. (Ch. 6); Harper's Illustrated Biochemistry (Ch. 36)_