DNA Polymerase 1 2 3: Functions and Roles in Replication

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to DNA Polymerases

What Are DNA Polymerases?

DNA polymerases are a family of enzymes that catalyze the template-directed synthesis of DNA from deoxyribonucleoside triphosphates (dNTPs). These enzymes are fundamental to all life, responsible for the accurate duplication of genomic material during cell division and for maintaining genome integrity through various DNA repair pathways. The reaction catalyzed by all DNA polymerases is the addition of a nucleotide to the 3′-hydroxyl (3′-OH) terminus of a growing DNA strand, with the template strand dictating which nucleotide is incorporated via Watson–Crick base pairing.

The general reaction can be written as:

DNAₙ + dNTP → DNAₙ₊₁ + PPᵢ

where PPᵢ is inorganic pyrophosphate. The energy driving this reaction comes from the hydrolysis of the high-energy phosphoanhydride bond between the α and β phosphates of the incoming dNTP. Importantly, all DNA polymerases share a fundamental requirement: they cannot initiate synthesis de novo. They require a pre-existing 3′-OH group provided by a primer—either an RNA primer synthesized by a primase or a DNA strand with a free 3′-OH. This is a critical constraint that shapes the entire mechanism of DNA replication, as discussed in Primase vs Polymerase.

In the bacterium Escherichia coli, three DNA polymerases—designated DNA Polymerase I (Pol I), DNA Polymerase II (Pol II), and DNA Polymerase III (Pol III)—were identified and characterized in the late 1950s and 1960s by Arthur Kornberg and colleagues. These three enzymes, while sharing a common catalytic core, have distinct structures, processivities, and physiological roles. Understanding the division of labor among Pol I, Pol II, and Pol III is essential for grasping how a single round of replication produces two complete daughter chromosomes with high fidelity.

Prokaryotic vs. Eukaryotic Polymerases

It is important to place the prokaryotic polymerases in a broader context. Eukaryotic cells contain at least 15 distinct DNA polymerases (Pol α, Pol δ, Pol ε, Pol β, Pol γ, Pol ζ, Pol η, Pol θ, Pol ι, Pol κ, Pol λ, Pol μ, Pol ν, Pol σ, and Pol τ), each with specialized roles in replication, repair, or both. The naming convention can be confusing: the eukaryotic replicative polymerases are Pol α, Pol δ, and Pol ε, which are functionally analogous to the prokaryotic Pol III, while Pol β and Pol λ function in base excision repair, analogous to some roles of prokaryotic Pol I.

The prokaryotic system, particularly E. coli, remains the best-understood model and is the focus of this article. The three E. coli polymerases are numbered in order of their discovery, not in order of their abundance or importance in replication. This historical numbering is a common source of confusion, as the most abundant polymerase (Pol I, ~400 molecules per cell) is not the primary replicative enzyme; that role belongs to Pol III, which is present at only ~10–20 molecules per cell but has far greater processivity and speed.

DNA Polymerase I: Structure and Function

Domains and Activities

DNA Polymerase I (Pol I) is a monomeric enzyme of 928 amino acids (approximately 103 kDa) encoded by the polA gene in E. coli. It is the most abundant DNA polymerase in the cell, with roughly 400 molecules per cell. Pol I is a multifunctional enzyme that possesses three distinct catalytic activities:

1. 5′→3′ polymerase activity: Adds nucleotides to a 3′-OH primer terminus.
2. 3′→5′ exonuclease activity: Removes nucleotides from the 3′ end of a DNA strand (proofreading).
3. 5′→3′ exonuclease activity: Removes nucleotides from the 5′ end of a DNA strand, including RNA primers.

The protein folds into three structural domains that correspond to these activities. The C-terminal domain (approximately 33 kDa) contains the 5′→3′ polymerase active site. The central domain (approximately 20 kDa) contains the 3′→5′ exonuclease active site. The N-terminal domain (approximately 35 kDa) contains the 5′→3′ exonuclease activity. The Klenow fragment, a well-known proteolytic cleavage product of Pol I, consists of the polymerase and 3′→5′ exonuclease domains but lacks the 5′→3′ exonuclease domain. This fragment has been widely used in molecular biology for DNA labeling and sequencing reactions.

The 5′→3′ exonuclease activity of Pol I is unique among the three E. coli polymerases. It recognizes a displaced 5′ end at a nick or a junction between a RNA primer and DNA, and it can remove nucleotides in a processive manner, degrading the strand ahead of the polymerase. This activity is essential for two processes: Okazaki fragment maturation and the removal of damaged DNA during base excision repair.

Role in DNA Repair

Pol I plays a central role in base excision repair (BER), the pathway that removes damaged bases such as uracil, 8-oxoguanine, and alkylated bases. The repair process proceeds as follows:

1. A DNA glycosylase recognizes and removes the damaged base, creating an apurinic/apyrimidinic (AP) site.
2. An AP endonuclease nicks the phosphodiester backbone 5′ to the AP site, creating a single-strand break with a 5′-deoxyribose phosphate (5′-dRP) moiety.
3. Pol I, via its 5′→3′ exonuclease activity, removes the 5′-dRP and a few downstream nucleotides.
4. Simultaneously, the polymerase activity of Pol I fills in the resulting gap using the intact complementary strand as a template.
5. DNA ligase seals the nick.

This "cut-and-patch" mechanism requires the coordinated action of the 5′→3′ exonuclease and polymerase activities of Pol I. The enzyme effectively performs nick translation—moving the nick downstream by simultaneously removing nucleotides ahead and adding nucleotides behind. This is mechanistically distinct from the role of DNA ligase, which only seals pre-existing nicks; the distinction is elaborated in Difference Between Ligase and DNA Polymerase 1.

Pol I also participates in the repair of UV-induced DNA damage and in the resynthesis step of nucleotide excision repair (NER), where it fills the ~12-nucleotide gap left after the damaged oligonucleotide is excised.

DNA Polymerase II: The Repair Enzyme

Induction and Regulation

DNA Polymerase II (Pol II) is encoded by the polB gene (also called dinA) in E. coli. It is a monomeric enzyme of 783 amino acids (approximately 90 kDa). Under normal growth conditions, Pol II is present at only ~30–50 molecules per cell, but its expression is strongly induced as part of the SOS response—a global transcriptional program activated by DNA damage. When DNA damage accumulates, the RecA protein becomes activated and promotes the autocleavage of the LexA repressor, leading to the derepression of more than 40 genes, including polB. Within minutes of DNA damage, Pol II levels can increase 5- to 10-fold.

Pol II is a B-family polymerase, structurally related to the eukaryotic replicative polymerases Pol α, Pol δ, and Pol ε, and to the bacteriophage T4 and T7 DNA polymerases. This is in contrast to Pol I and Pol III, which belong to the A-family and C-family, respectively. Pol II possesses both 5′→3′ polymerase activity and 3′→5′ exonuclease proofreading activity, but it lacks the 5′→3′ exonuclease domain found in Pol I.

Translesion Synthesis

The primary function of Pol II is in DNA repair, particularly in the restart of replication forks that have stalled at sites of DNA damage. Pol II is involved in two distinct repair processes:

1. Replication restart: When a replication fork encounters a lesion in the template strand, the replicative polymerase (Pol III) stalls. The fork can regress, forming a four-way junction (chicken-foot structure). Pol II is recruited to this structure and can perform limited DNA synthesis to allow fork regression and restart. This process requires the coordinated action of several proteins, including RecA and the PriA replication restart proteins.

1. Translesion synthesis (TLS): Pol II can bypass certain types of DNA damage that block Pol III. Unlike Pol I and Pol III, which are highly accurate and stall at damaged bases, Pol II has a more flexible active site that can accommodate distorted template structures. Pol II can efficiently bypass abasic sites and some bulky adducts, although with reduced fidelity. Importantly, Pol II inserts nucleotides opposite damaged sites with relatively high accuracy compared to the specialized TLS polymerases (Pol IV and Pol V in E. coli), which are error-prone by design.

Pol II also plays a role in adaptive mutagenesis—the process by which mutations accumulate in stressed cells—and in the repair of cross-linked DNA. Its low processivity (it synthesizes only ~10–50 nucleotides before dissociating) is well-suited for these repair functions, where short patches of synthesis are required.

DNA Polymerase III: The Replicative Workhorse

Holoenzyme Composition

DNA Polymerase III (Pol III) is the enzyme responsible for the bulk of chromosomal DNA replication in E. coli. It is a large, multi-subunit complex with a molecular mass of approximately 900 kDa. The holoenzyme is composed of 10 different subunits, each with a specific function:

The α, ε, and θ subunits form the catalytic core. The α subunit contains the polymerase active site, while the ε subunit provides the proofreading 3′→5′ exonuclease activity. The θ subunit is a small accessory protein that stabilizes the ε subunit. The τ and γ subunits are both encoded by the dnaX gene; τ is the full-length protein, while γ is produced by a programmed ribosomal frameshift that generates a truncated protein lacking the C-terminal domain responsible for dimerization and helicase binding.

The holoenzyme assembles as a dimer, with two catalytic cores held together by τ subunits. This dimeric architecture allows the simultaneous synthesis of both the leading and lagging strands at the replication fork. The γ complex (comprising γ, δ, δ′, χ, and ψ) functions as the clamp loader, which loads the β sliding clamp onto DNA.

Processivity and Sliding Clamp

The defining feature of Pol III is its remarkable processivity—the ability to synthesize long stretches of DNA without dissociating from the template. The free catalytic core (αεθ) has a processivity of only ~10–20 nucleotides. However, when the β clamp is loaded onto the DNA, the processivity increases to greater than 500,000 nucleotides. This dramatic enhancement is achieved through a topological mechanism: the β clamp is a ring-shaped homodimer that encircles the DNA duplex and slides freely along it. The clamp tethers the polymerase to the template, preventing dissociation.

The β clamp is loaded onto DNA by the γ complex in an ATP-dependent reaction:

1. The γ complex binds ATP and adopts a conformation that opens the β clamp ring.
2. The γ complex–β clamp assembly recognizes a primer–template junction.
3. ATP hydrolysis triggers the release of the β clamp around the DNA, and the γ complex dissociates.
4. The αεθ core binds to the β clamp, and processive synthesis begins.

The β clamp is functionally analogous to the eukaryotic proliferating cell nuclear antigen (PCNA) and the bacteriophage T4 gp45 protein. The clamp not only increases processivity but also serves as a platform for recruiting other proteins involved in replication, repair, and cell cycle regulation.

Pol III synthesizes DNA at a rate of approximately 1,000 nucleotides per second, which is sufficient to replicate the 4.6-megabase E. coli chromosome in approximately 40 minutes. This speed is roughly 10–20 times faster than Pol I and reflects the demands of replicating an entire genome within the bacterial cell cycle.

At the replication fork, Pol III functions as a dimeric complex. The leading strand is synthesized continuously by one core, while the lagging strand is synthesized discontinuously as Okazaki fragments by the other core. The lagging-strand core must repeatedly dissociate and reassociate with new primers as each Okazaki fragment is initiated. This cyclical process is coordinated by the τ subunits, which hold the two cores together and also bind the DnaB helicase, ensuring that the leading and lagging strand synthesis remain coupled. The role of the helicase in unwinding the duplex is distinct from that of the polymerase, as detailed in Helicase vs Polymerase.

Comparison of Polymerases 1, 2, and 3

Key Differences at a Glance

The three E. coli DNA polymerases differ substantially in their properties and functions. The following table summarizes the key differences:

A more detailed comparison of Pol I and Pol III is available in Difference Between DNA Polymerase 1 and 3 and Difference Between DNA Polymerase I and Iii.

Several points merit emphasis. First, Pol III is the only polymerase with sufficient processivity and speed to replicate an entire chromosome. Second, Pol I is the only polymerase with 5′→3′ exonuclease activity, which is essential for removing RNA primers. Third, Pol II is the only SOS-inducible polymerase among the three and is specialized for repair rather than replication.

Mechanism of Nucleotide Addition

Catalytic Cycle

All DNA polymerases, regardless of family or organism, catalyze nucleotide addition through a conserved two-metal-ion mechanism. The reaction occurs in the polymerase active site, which contains two conserved aspartate residues that coordinate two Mg²⁺ ions. The catalytic cycle proceeds as follows:

1. Binding: The polymerase binds to a primer–template junction. The template strand positions the next template base in the active site.

1. dNTP selection: The incoming dNTP enters the active site and forms a Watson–Crick base pair with the template base. The polymerase undergoes a conformational change (from an open to a closed state) that encloses the dNTP and checks the geometry of the base pair. This induced-fit mechanism contributes significantly to fidelity—incorrect base pairs have distorted geometry and are rejected before chemistry occurs.

1. Nucleophilic attack: The 3′-OH of the primer terminus performs a nucleophilic attack on the α-phosphate of the incoming dNTP. One Mg²⁺ ion (metal A) lowers the pKa of the 3′-OH and stabilizes the developing negative charge, while the second Mg²⁺ ion (metal B) coordinates the β and γ phosphates and stabilizes the pyrophosphate leaving group.

1. Pyrophosphate release: The reaction forms a phosphodiester bond and releases inorganic pyrophosphate (PPᵢ). The polymerase returns to the open conformation and translocates one nucleotide along the template, exposing the next template base for the subsequent round of addition.

1. Translocation: The polymerase moves forward by one base pair, and the cycle repeats.

The overall error rate of DNA synthesis by Pol III is approximately 10⁻⁷ to 10⁻⁸, which reflects the combined contributions of base selection (~10⁻⁵), induced-fit discrimination (~10⁻³), and proofreading (~10⁻² to 10⁻³).

Proofreading by 3′→5′ Exonuclease

The 3′→5′ exonuclease activity provides a second chance to correct errors. When a mismatched nucleotide is incorporated, the polymerase stalls because the mismatched base pair has distorted geometry and cannot be extended efficiently. The primer terminus then "frayers" and can transfer from the polymerase active site to the exonuclease active site, which is located approximately 30–40 Å away in Pol I and in a separate subunit (ε) in Pol III.

The exonuclease active site has a different set of conserved residues that coordinate two Mg²⁺ ions in a manner similar to the polymerase active site, but with a geometry that accommodates single-stranded DNA. The mismatched nucleotide is removed by hydrolysis of the phosphodiester bond, releasing a 5′-monophosphate. The shortened primer then reanneals to the template and is transferred back to the polymerase active site for continued synthesis.

This proofreading mechanism is highly efficient. The 3′→5′ exonuclease increases the overall fidelity of DNA replication by approximately 100-fold. Defects in proofreading are associated with human disease, including a hereditary cancer predisposition syndrome known as Polymerase Proofreading Associated Polyposis, which results from mutations in the exonuclease domains of the eukaryotic replicative polymerases Pol δ and Pol ε.

Experimental Methods to Study DNA Polymerases

In Vitro Assays

Several classical assays are used to characterize DNA polymerase activity:

Primer extension assay: A radiolabeled or fluorescently labeled primer is annealed to a template, and the polymerase is added in the presence of dNTPs. The reaction products are separated by denaturing polyacrylamide gel electrophoresis and visualized by autoradiography or fluorescence imaging. This assay reveals the processivity of the polymerase (the distribution of product lengths) and the rate of synthesis.

Steady-state kinetics: The rate of nucleotide incorporation is measured as a function of dNTP concentration. The Michaelis–Menten parameters (Kₘ and k_cat) are determined for correct and incorrect nucleotides. The fidelity of nucleotide selection is quantified as the ratio (k_cat/Kₘ) for correct versus incorrect dNTPs.

Exonuclease assays: A radiolabeled primer with a mismatched 3′ terminus is incubated with the polymerase, and the release of mononucleotides is monitored. This assay measures the proofreading activity.

Single-molecule assays: Optical tweezers or fluorescence resonance energy transfer (FRET) can be used to observe individual polymerase molecules in real time. These techniques have revealed the dynamics of polymerase translocation, clamp loading, and proofreading.

Typical in vitro reaction conditions for E. coli Pol III include 20 mM Tris-HCl (pH 7.5), 8 mM MgCl₂, 50 mM potassium glutamate, 1 mM dithiothreitol, 100 µg/mL bovine serum albumin, and 100 µM each dNTP, incubated at 37°C. The reaction is quenched with EDTA and formamide before gel analysis.

Genetic Approaches

Genetic studies have been instrumental in assigning functions to the three polymerases. The polA gene was the first to be studied; polA mutants were isolated by their sensitivity to ultraviolet light and alkylating agents. These mutants are viable but grow slowly and have defects in Okazaki fragment maturation, confirming Pol I's role in this process.

The dnaE gene (encoding the α subunit of Pol III) was identified through temperature-sensitive mutants that fail to replicate DNA at the non-permissive temperature (42°C). These mutants arrest replication within seconds of the temperature shift, demonstrating that Pol III is the replicative polymerase.

The polB gene was identified through its homology to other B-family polymerases and through its induction by DNA damage. polB mutants are viable and show only mild phenotypes under normal conditions, but they are defective in the restart of replication after UV-induced damage and in adaptive mutagenesis.

Combinatorial approaches, such as constructing strains with mutations in multiple polymerase genes, have revealed the functional redundancy and specialization of the three enzymes. For example, polA polB double mutants are viable but show increased sensitivity to DNA damage, indicating overlapping roles in repair.

Common Misconceptions and Pitfalls

Pol I vs. Pol III

The most common error students make is assuming that the most abundant polymerase (Pol I) is the main replicative enzyme. This is incorrect. Pol III is the replicative polymerase, despite being present at only ~10–20 molecules per cell. Pol I is more abundant because it is needed at every Okazaki fragment (there are ~4,000 per chromosome) and because it participates in repair throughout the genome. Pol I's high abundance compensates for its low processivity.

A related misconception is that Pol I synthesizes the leading strand. In fact, Pol III synthesizes both the leading and lagging strands. Pol I's role is limited to removing RNA primers and filling the resulting gaps during Okazaki fragment maturation.

Proofreading Direction

Students often confuse the directionality of the exonuclease activities. The 3′→5′ exonuclease removes nucleotides from the 3′ end of the growing strand—this is proofreading. The 5′→3′ exonuclease removes nucleotides from the 5′ end of a strand—this is used for RNA primer removal and nick translation. Only Pol I has the 5′→3′ exonuclease activity among the three E. coli polymerases.

Another common error is thinking that proofreading occurs after the polymerase has moved on. In reality, proofreading occurs immediately after a misincorporation, before the next nucleotide is added. The polymerase detects the mismatch through geometric distortion and transfers the primer to the exonuclease active site.

Prokaryotic vs. Eukaryotic Polymerases

Students often transfer their knowledge of prokaryotic polymerases to eukaryotic systems incorrectly. The eukaryotic replicative polymerases are Pol α, Pol δ, and Pol ε, not Pol I, II, and III. The numbering systems are entirely different. Furthermore, eukaryotic cells have many more polymerases with specialized functions, and the rules governing their activities differ. For example, eukaryotic Pol α has primase activity, which is not found in any of the three E. coli polymerases.

Frequently Asked Questions

What is the main function of DNA polymerase 1, 2, and 3?

DNA Polymerase I removes RNA primers and fills the resulting gaps during Okazaki fragment maturation, and it participates in base excision repair. DNA Polymerase II is primarily a repair enzyme that is induced by DNA damage and functions in replication restart and translesion synthesis. DNA Polymerase III is the main replicative polymerase responsible for synthesizing both the leading and lagging strands of the E. coli chromosome.

Which DNA polymerase is responsible for the majority of DNA replication in E. coli?

DNA Polymerase III is responsible for the majority of DNA replication in E. coli. It synthesizes both the leading and lagging strands at the replication fork at a rate of approximately 1,000 nucleotides per second. Pol I contributes only a small fraction of total synthesis (the short gaps left after RNA primer removal), and Pol II contributes essentially nothing to normal replication.

What is the difference between DNA polymerase 1 and 3?

The key differences are processivity, speed, and function. Pol I has low processivity (~20–100 nucleotides), synthesizes at ~20 nucleotides per second, and functions in Okazaki fragment maturation and repair. Pol III has extremely high processivity (>500,000 nucleotides with the β clamp), synthesizes at ~1,000 nucleotides per second, and is the primary replicative enzyme. Pol I also possesses a unique 5′→3′ exonuclease activity that Pol III lacks, while Pol III is a multi-subunit complex with a dimeric architecture that coordinates leading and lagging strand synthesis.

Does DNA polymerase 2 have a role in replication?

DNA Polymerase II does not participate in normal chromosomal replication. Its primary roles are in DNA repair, particularly in the restart of stalled replication forks and in translesion synthesis past certain types of DNA damage. Pol II is induced as part of the SOS response and is important for survival after DNA damage, but it is not required for normal growth.

What is the function of the 3' to 5' exonuclease activity in DNA polymerases?

The 3′→5′ exonuclease activity provides proofreading. When a polymerase misincorporates a nucleotide, the mismatched base pair has distorted geometry that stalls further synthesis. The primer terminus is then transferred to the exonuclease active site, where the mismatched nucleotide is removed. This activity increases the fidelity of DNA replication by approximately 100-fold, reducing the error rate from ~10⁻⁵ to ~10⁻⁷.

Why is DNA polymerase 3 faster than DNA polymerase 1?

Pol III is faster because it has evolved to replicate an entire chromosome within the bacterial cell cycle. Its high speed (~1,000 nucleotides per second) is enabled by the β sliding clamp, which tethers the polymerase to the template and prevents dissociation. The clamp allows Pol III to synthesize long stretches of DNA without pausing. Pol I, in contrast, only needs to fill short gaps (~10–30 nucleotides) during Okazaki fragment maturation and repair, so high speed is not required. Pol I's slower speed (~20 nucleotides per second) is sufficient for its limited synthetic tasks.

Which DNA polymerase removes RNA primers?

DNA Polymerase I removes RNA primers in E. coli. Its 5′→3′ exonuclease activity degrades the RNA primer starting from its 5′ end, while the polymerase activity simultaneously fills the gap with DNA. This combined action is called nick translation. In eukaryotic cells, RNA primers are removed by a different mechanism involving the flap endonuclease FEN1 and the nuclease Dna2, in coordination with Pol δ and Pol ε.

Key Takeaways

• E. coli has three DNA polymerases: Pol I (repair and primer removal), Pol II (SOS-induced repair), and Pol III (replicative polymerase).
• Pol III is the main replicative enzyme, synthesizing both leading and lagging strands at ~1,000 nucleotides per second with processivity exceeding 500,000 nucleotides.
• Pol I is the only polymerase with 5′→3′ exonuclease activity, which is essential for removing RNA primers during Okazaki fragment maturation.
• All three polymerases possess 3′→5′ exonuclease proofreading activity, which increases replication fidelity by ~100-fold.
• The β sliding clamp is essential for Pol III processivity; without it, Pol III synthesizes only ~10–20 nucleotides before dissociating.
• The two-metal-ion mechanism of nucleotide addition is conserved across all DNA polymerases, from bacteria to humans.
• Pol II is induced by the SOS response and functions in replication restart and translesion synthesis, not in normal replication.
• The numbering of the polymerases reflects their order of discovery, not their importance in replication; Pol III, not Pol I, is the replicative workhorse.

Further Reading

• Dauphin LA et al. Comparative evaluation of automated and manual commercial DNA extraction methods for detection of Francisella tularensis DNA from suspensions and spiked swabs by real-time polymerase chain reaction. Diagnostic microbiology and infectious disease. 2011. PubMed 21546201
• Mandir AS et al. Poly(ADP-ribose) polymerase activation mediates 1-methyl-4-phenyl-1, 2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism. Proceedings of the National Academy of Sciences of the United States of America. 1999. PubMed 10318960
• Semizarov DG et al. Modified nucleoside 5'-triphosphates containing 2',3'-fused three-membered rings as substrates for different DNA polymerases. FEBS letters. 1993. PubMed 768756581036-y)

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