DNA Polymerase: Structure, Types, and Functions

Every time a cell divides, it must copy about 3 billion base pairs of DNA, and it must copy them almost perfectly. The enzyme that does this building work is DNA polymerase. It reads an existing DNA strand and lays down a new one base by base, matching each new nucleotide to the template. It is one of the most important enzymes in biology, because without it a cell cannot pass a correct copy of its genes to the next generation.

DNA polymerase does more than build. It also checks its own work as it goes, catching and removing most of its mistakes. This combination of speed and accuracy is what makes reliable inheritance possible, and it is also what makes DNA polymerase so useful in the laboratory, where a heat-stable version of it powers the PCR test used in diagnosis and research. This article explains what DNA polymerase is, how it is built, the different types found in cells, and what it does.

What is DNA polymerase?

DNA polymerases also possess proofreading capabilities to correct errors that may occur during DNA synthesis, contributing to the overall fidelity of DNA replication and the maintenance of genetic integrity.

DNA polymerase is an enzyme that makes new DNA by joining nucleotides together in a set order. It builds a new strand using an existing strand as a pattern, following the base-pairing rules: adenine pairs with thymine, and guanine pairs with cytosine. So the sequence of the new strand is decided by the template.

Three simple rules describe how every DNA polymerase works, and they explain most of what follows:

It builds in one direction only. DNA polymerase adds each new nucleotide to the 3′ end of the growing strand, so the new strand always grows in the 5′ to 3′ direction. It cannot build the other way.

It cannot start on its own. DNA polymerase can only add to an existing end. It needs a short starter, called a primer, to give it a free 3′ end to build from. The primer is made by a different enzyme (primase).

It checks its work. Many DNA polymerases proofread as they build, removing a wrong nucleotide before moving on. This is why DNA replication makes so few errors.

Beyond copying DNA during cell division, DNA polymerase also repairs damaged DNA and takes part in recombination. The full role of DNA polymerase at the replication fork, alongside the other enzymes, is covered in the articles on DNA replication and the enzymes involved in DNA replication.

Structure of DNA Polymerase

DNA polymerase has a shape that is famously compared to a right hand. This is not just a memory trick. The main parts of the enzyme are named after the parts of a cupped hand, because they sit in the same arrangement and grip the DNA the way a hand would hold a thread. The three key regions are the fingers, the palm, and the thumb, and each has a job. Understanding the hand shape makes the rest of the structure easy to remember.

1. Palm: holds the active site, where the new phosphodiester bond is actually made. This is the catalytic center.
2. Fingers: recognize the incoming nucleotide and check that it matches the template base before it is added.
3. Thumb: grips the newly made DNA and helps hold the enzyme on the strand as it moves.
4. Exonuclease Domain (Proofreading): Some DNA polymerases have an exonuclease domain responsible for proofreading the newly synthesized DNA strand. This domain can remove incorrectly incorporated nucleotides by excising them from the 3′ end of the growing DNA chain, improving the overall fidelity of DNA replication.
5. Accessory Proteins: Besides the core catalytic domains, DNA polymerases often interact with accessory proteins that assist in various aspects of DNA replication, such as processivity, primer recognition, and coordination with other replication machinery components.

Overall, the structure of this is highly specialized to perform its essential functions in DNA replication and repair accurately and efficiently. Variations in structure and function exist among different DNA polymerase types, reflecting their diverse roles in maintaining genomic integrity across different organisms.

Types of DNA Polymerase

There are several types of polymerases involved in various DNA-related processes, including DNA replication, repair, and synthesis. The types of DNA polymerase vary among prokaryotes and Eukaryotes.

Types of DNA Polymerase Found in Prokaryotes

Prokaryotic organisms, such as bacteria, have several DNA polymerases that play different roles in DNA replication, repair, and other cellular processes.

1. DNA Polymerase I (Pol I): Functions in DNA repair and gap filling during DNA synthesis. It has 5′ to 3′ polymerase activity for DNA synthesis. This type also possesses a 3′ to 5′ exonuclease (proofreading) activity, which helps correct errors during replication. It also assists in removing RNA primers during Okazaki fragment maturation in DNA replication.
2. DNA Polymerase II (Pol II): It is involved in DNA repair, especially repair that responds to DNA damage from environmental factors such as UV light. It exhibits 5′ to 3′ polymerase activity and lacks 3′ to 5′ exonuclease activity.
3. DNA Polymerase III (Pol III): It is prokaryotes’ primary DNA replication enzyme, including bacteria like Escherichia coli (E. coli). The highly processive enzyme is responsible for producing the leading as well as lagging strands during DNA replication. It is made of multiple subunits, including the core catalytic subunit responsible for DNA synthesis.
   
   The two most important prokaryotic polymerases to know are Pol III, the main enzyme that builds most of the new DNA, and Pol I, which removes the RNA primers and fills the gaps. Pol II, IV, and V are mainly repair and damage-tolerance enzymes.
4. DNA Polymerase IV (Pol IV) and DNA Polymerase V (Pol V): These are specialized DNA polymerases involved in translesion DNA synthesis, particularly when regular DNA polymerases encounter DNA lesions or damage. Pol IV and Pol V are error-prone and can replicate past damaged sites with reduced fidelity, allowing cells to tolerate DNA damage temporarily.

These DNA polymerases in prokaryotes work together in a coordinated manner during DNA replication, repair, and other DNA-related processes to ensure accurate DNA synthesis, maintain genomic stability, and respond to DNA damage. Each type has specific functions and characteristics that contribute to the fidelity and efficiency of DNA replication and repair in prokaryotic cells.

Types of DNA Polymerase Found in Eukaryotes

Eukaryotic organisms, like plants, animals, fungi, and protists, possess a variety of DNA polymerases that are required in different aspects of DNA replication, repair, and maintenance. The five to focus on are Pol α (starts replication), Pol δ (lagging strand), Pol ε (leading strand), Pol β (base excision repair), and Pol γ (mitochondrial DNA). The rest are specialized repair and damage-tolerance enzymes.

1. DNA Polymerase α (Pol α): Starts DNA replication. In eukaryotes, Pol α exists as a complex with primase: the primase makes a short RNA primer, and Pol α extends it with a short piece of DNA, producing an RNA-DNA primer on both the leading and lagging strands. It works with other replication proteins during the initiation of replication.
2. DNA Polymerase δ (Pol δ): It is the main replication enzyme for the lagging strand. It exhibits high processivity and fidelity. Involved in synthesizing Okazaki fragments on the lagging strand.
3. DNA Polymerase ε (Pol ε): It is the main replication enzyme for the leading strand. Highly processive and accurate. Works in coordination with other replication proteins to synthesize the leading strand continuously.
4. DNA Polymerase β (Pol β): Involved in base excision repair (BER) pathway. This polymerase specializes in filling the gaps after damaged bases are removed during BER.
5. DNA Polymerase γ (Pol γ): It is present in mitochondria (mitochondrial DNA polymerase). Responsible for replicating and maintaining the mitochondrial genome. This type of polymerase plays a crucial role in mitochondrial DNA repair processes.
6. Translesion synthesis (TLS) polymerases (Pol η, κ, ι, ζ, and Rev1): This is a specialized group that can copy past damaged spots in the DNA when the normal polymerases stall. They do this at the cost of accuracy, so they are error-prone. Their value is that they let replication continue rather than stop at a lesion. For most purposes, it is enough to know this group exists and works in damage tolerance, rather than to memorize each one.
   
   These eukaryotic DNA polymerases have diverse functions and specialize in DNA replication and repair pathways. They contribute to maintaining genomic stability, accurately replicating DNA, and repairing damaged DNA to ensure the integrity of the genetic information in eukaryotic cells.

Functions

Figure: Diagram of DNA polymerase extending a DNA strand and proofreading

DNA polymerase’s primary function is to act as a catalyst during the synthesis of new DNA strands by adding new nucleotides to the growing DNA chain during DNA replication, repair, and recombination processes.

1. DNA Replication: This enzyme plays a central role in DNA replication, where it copies the entire genome of a cell before cell division. During replication, DNA polymerase synthesizes new DNA strands complementary to the template strands. The leading strand synthesizes continuously in the 5′ to 3′ direction. However, the lagging strand synthesizes discontinuously in Okazaki fragments.
2. Nucleotide Addition: It catalyzes the addition of deoxyribonucleotides (dNTPs) to the 3′ end of the growing DNA strand. It forms phosphodiester bonds between the incoming nucleotide and the last nucleotide of the ever-increasing strand, extending the DNA chain.
3. Proofreading: Many DNA polymerases possess proofreading capabilities to maintain high fidelity during DNA synthesis. They have 3′ to 5′ exonuclease activity, allowing them to find and correct errors made during replication by removing incorrectly incorporated nucleotides from the 3′ end of the growing strand.
4. Processivity: DNA polymerases are highly processive enzymes that can add multiple nucleotides sequentially without dissociating from the DNA template. This processivity ensures efficient DNA synthesis during replication and repair processes.
5. DNA Repair: In addition to replication, DNA polymerases assist in DNA repair mechanisms. For example, during base excision repair, specialized DNA polymerases help fill the gaps left after the removal of damaged bases. Similarly, DNA polymerases are involved in nucleotide excision repair and mismatch repair pathways to correct various types of DNA damage and mismatches.
6. Translesion Synthesis: Some DNA polymerases are specialized in bypassing DNA lesions or damage during replication. These polymerases, known as translesion polymerases, can replicate past damaged sites in the DNA template, albeit with reduced fidelity, to prevent replication stalling and maintain genome integrity.

DNA polymerase in the laboratory: PCR and sequencing

DNA polymerase is not only a cellular enzyme. It is one of the most important tools in the molecular biology laboratory, because if you give a purified DNA polymerase a template, primers, and free nucleotides, it will copy DNA in a test tube.

The best-known example is the polymerase chain reaction (PCR), the method used to make millions of copies of a target DNA sequence. PCR is central to diagnosis (for example, detecting the DNA or RNA of a pathogen), forensic testing, and research. PCR needs a DNA polymerase that can survive the high temperature used to separate the two DNA strands in each cycle. An ordinary polymerase would be destroyed by that heat. The solution is Taq polymerase, taken from a heat-tolerant bacterium, Thermus aquaticus, that lives in hot springs. Because Taq polymerase is stable at high temperature, it keeps working cycle after cycle, which is what makes PCR practical.

DNA polymerases are also the engine of DNA sequencing. In the widely used sequencing methods, a DNA polymerase builds a new strand while the machine records which base is added at each step, reading the sequence as it goes. The full method is covered in the articles on PCR and on DNA sequencing.

How to Remember

The right hand. DNA polymerase is shaped like a cupped right hand. The palm holds the active site (where the work is done), the fingers pick and check the incoming nucleotide, and the thumb grips the finished DNA. Picture holding a thread in your hand: palm underneath, fingers closing over the new piece, thumb steadying it.

The two rules it can never break. DNA polymerase builds only 5′ to 3′, and it can never start from scratch (it always needs a primer). Almost every exam trick about polymerase comes back to these two rules.

Pol I versus Pol III (prokaryotes). The higher number does the bigger job: Pol III builds most of the new DNA. Pol I is the cleanup enzyme that removes the RNA primer and fills the gap. If the question is about primer removal, the answer is the lower number.

The eukaryotic core five, by location and job. α starts, δ does the lagging strand, ε does the leading strand, β repairs (base excision), γ handles mitochondrial DNA. A phrase: "α starts, δ lags, ε leads, β repairs, γ is mitochondrial."

Taq for PCR. The polymerase used in PCR is Taq, from a bacterium in hot springs, because it survives heat. "Hot spring bug, hot PCR" links the two.

Key exam facts in one table

Where Students Get Confused

DNA polymerase cannot start a new strand. It can only extend an existing 3′ end, so it always needs a primer to begin. Students often assume the polymerase starts replication itself; it does not. Primase makes the starter.

Direction: 5′ to 3′ synthesis, but it reads the template 3′ to 5′. The new strand grows 5′ to 3′. The template is read in the opposite direction. Both statements are true and describe the same event from different ends; they are not a contradiction.

Pol I and Pol III are easy to swap. Both are prokaryotic, both build DNA, both proofread. Pol III is the main builder; Pol I removes RNA primers and fills gaps. Match "main synthesis" to III and "primer removal" to I.

Prokaryotic and eukaryotic polymerases do not match by number or letter. Bacterial Pol III is the main replicase; in eukaryotes that job is split between Pol δ and Pol ε. Do not try to line up "III" with any single Greek letter.

Proofreading is 3′ to 5′; primer removal (by Pol I) is 5′ to 3′. Both are exonuclease activities, so they get confused. Proofreading trims a wrong base just added at the 3′ end (so 3′ to 5′). Primer removal chews from the 5′ end of the primer (so 5′ to 3′)

References

1. Watson JD, Baker TA, Bell SP, Gann A, Levine M, Losick R. Molecular Biology of the Gene. 7th ed. Pearson; 2013.
2. Nelson DL, Cox MM. Lehninger Principles of Biochemistry. 8th ed. W.H. Freeman; 2021.
3. Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell. 7th ed. W.W. Norton; 2022.
4. Steitz TA. DNA polymerases: structural diversity and common mechanisms. J Biol Chem. 1999;274(25):17395-17398. https://doi.org/10.1074/jbc.274.25.17395