What is DNA Polymerase?
DNA Polymerase belongs to the family of enzymes that initiate the synthesis of DNA molecules in the body by assembling the nucleotides, known as the building blocks of DNA or Deoxyribonucleic acid. DNA polymerase acts as an essential enzyme for DNA replication in the body. The enzymes work in a group to create two equivalent DNA duplexes from a single strand of the original DNA duplex. During the process of replicating the DNA duplexes, these enzymes read and inspect the original DNA strand to create two replicas that match the original strand of DNA.
The enzyme act as a catalyst in the chemical reaction to replicate DNA duplexes. Now, let’s discuss the functions, structure, and types of DNA polymerase.
History of DNA Polymerase
The history of DNA polymerase can be traced back to 1956 when Arthur Kornberg and his colleagues discovered the enzyme in Escherichia Coli. Arthur Kornberg named the enzyme as DNA Polymerase I and described the DNA replication process, where these enzymes were involved in creating two replica DNA strands from the original. Later in 1970, Thomas Kornberg (son of Arthur Kornberg) and his colleague Malcolm E. Gefter discovered DNA polymerase II. They also described the role of DNA polymerase I and II in DNA replication.
DNA Polymerase Structure
All the known DNA Polymerases have highly conserved structures, which means that each catalytic subunit of the enzyme varies from the other in different species, independent of their original structures. The highly conserved DNA polymerase structure indicates the essential and irreplaceable functions of the cells. We can describe the shape of the DNA polymerase as the right hand with the only thumb, finger, and palm domains.
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DNA Polymerase Function
We can divide the role of DNA Polymerase under three significant heads. Let’s discuss each head in detail.
Replication of DNA: The primary function of every DNA polymerase enzyme is to initiate the DNA synthesis and replication in organisms by assembling the nucleotides. These enzymes act as a catalyst in the chemical process of DNA replication. The DNA polymerase enzymes work in clusters or groups to replicate the original DNA strand into two DNA duplexes.
Repairing: The replication of the DNA strand is a critical function, and it helps maintain the originality of the Genome. During the process of replication, the DNA strand incurs much damage. The DNA polymerase continuously repairs and rectifies the error in each DNA strand to maintain its integrity.
Proofreading: Proofreading is another crucial role of DNA Polymerase, where the enzyme checks all the nucleotides being added to duplicate the DNA strand. During the replication process, many errors occur in the replication of nucleotides, and each incorrect nucleotide should be removed to maintain the integrity of the genome. Hence, the DNA polymerase inspects each nucleotide and eliminates the incorrect ones.
DNA Polymerase Types
The DNA Polymerase types can be divided into two groups - The Prokaryotic and the Eukaryotic. Each group and polymerase enzymes differ in their structure and functions. Let’s discuss each type and its functions.
Prokaryotic DNA Polymerase Types
There are generally five types of Prokaryotic DNA Polymerase that were identified in the Escherichia Coli. Below we have mentioned all the types with their functions.
DNA Polymerase I: Polymerase I is coded by pol A genome. It consists of a single strand of polypeptide and plays an essential role in the recreation and repairs of the nucleotide during the replication process.
DNA Polymerase II: This polymerase is coded by the pol B genome. It consists of 7 catalytic subunits and helps in the repair of nucleotides during the replication process. The polymerase also backs up the DNA Polymerase III.
DNA Polymerase III: It is considered the primary enzyme for the replication process in Escherichia Coli. The polymerase III is coded by the pol C genome. It consists of 13 main catalytic subunits.
DNA Polymerase IV: Polymerase IV is coded by the din B genome and is primarily responsible for the DNA repair during SOS response.
DNA Polymerase V: Polymerase V is the primary enzyme involved in the SOS response and DNA repair during the replication process.
Eukaryotic DNA Polymerase Types
Like the prokaryotic cells, eukaryotic cells also contain DNA polymerase that helps synthesise and replicate DNA. Below, we have mentioned the crucial Eukaryotic DNA polymerase types:
DNA Polymerase Delta 𝛅: The polymerase delta is the primary enzyme for DNA replication in the eukaryotes, and it is involved in the proofreading process during DNA replication.
DNA Polymerase 𝜶: It is another vital polymerase in the eukaryotes and helps synthesise primer for the Okazaki fragments.
DNA Polymerase ∈: The polymerase ∈ is primarily involved in repairing the DNA strands and removes primer from the lagging strands.
DNA Polymerase ү: The polymerase ү acts as the main replicating enzyme for mitochondrial DNA replication.
Above, we discussed DNA polymerase, its structure, types, and functions, and we hope it would help gain in-depth knowledge about the subject. You can also read a DNA Polymerase ppt for more details.
FAQs on DNA Polymerase
1. What is DNA polymerase and what is its primary role in a cell?
DNA polymerase is a crucial enzyme that catalyses the synthesis of new DNA molecules from deoxyribonucleotides, the building blocks of DNA. Its primary role is DNA replication, where it reads an existing DNA strand as a template to create a new, complementary strand. This process ensures that genetic information is accurately copied before cell division.
2. What are the three main functions of DNA polymerase?
DNA polymerase has three principal functions that are essential for the life of an organism:
- Polymerisation: Its core function is to add nucleotides one by one to the growing DNA chain in a 5' to 3' direction, following the template strand.
- Proofreading: It has a 3' to 5' exonuclease activity, which allows it to check for and remove incorrectly paired nucleotides, thereby maintaining high fidelity during replication.
- Repair: It participates in various DNA repair mechanisms to correct damage that may occur to the DNA spontaneously or due to environmental factors.
3. What are the major types of DNA polymerase in prokaryotes and eukaryotes?
DNA polymerases differ between prokaryotic and eukaryotic cells:
- In prokaryotes (like E. coli), the main types are DNA Polymerase I, II, and III. DNA Polymerase III is the primary enzyme responsible for replication, while DNA Polymerase I is involved in removing RNA primers and DNA repair.
- In eukaryotes, there are several types, with the most important being Pol α, Pol δ (delta), and Pol ε (epsilon). Pol δ and Pol ε are the main replicative enzymes for the lagging and leading strands, respectively, while Pol α helps in initiating replication.
4. How does the structure of DNA polymerase relate to its function?
The structure of DNA polymerase is often compared to a right hand, with distinct domains for different functions. The 'palm' domain contains the active site where the polymerisation reaction occurs. The 'fingers' domain helps in positioning the correct incoming nucleotide opposite the template strand. The 'thumb' domain holds the template DNA strand in place, ensuring the enzyme remains attached during replication. This specialised structure allows for both high speed and high accuracy.
5. Why can DNA polymerase only synthesise DNA in the 5' to 3' direction?
This directionality is based on the chemistry of the polymerisation reaction. DNA polymerase catalyses the formation of a phosphodiester bond by adding the 5'-phosphate group of an incoming nucleotide to the free 3'-hydroxyl (-OH) group of the last nucleotide on the growing strand. There is no free hydroxyl group at the 5' end to which a new nucleotide can be attached, which restricts synthesis to the 5' to 3' direction only.
6. Beyond cell replication, what is the importance of DNA polymerase in biotechnology?
DNA polymerase is a fundamental tool in biotechnology, most notably in the Polymerase Chain Reaction (PCR). In PCR, a heat-stable DNA polymerase, such as Taq polymerase, is used to amplify a specific segment of DNA millions of times in a test tube. This technique is vital for applications like genetic testing, disease diagnosis, forensic science (DNA fingerprinting), and scientific research.
7. What is the key difference between the roles of DNA polymerase I and DNA polymerase III in E. coli?
The key difference lies in their primary roles during replication. DNA polymerase III is the main replicative enzyme; it synthesises the new DNA strands at a very high speed. In contrast, DNA polymerase I acts as a 'clean-up' enzyme. Its main job is to remove the RNA primers (which are needed to start synthesis) using its 5'→3' exonuclease activity and replace them with DNA nucleotides, filling in the gaps.
8. How does the 'proofreading' ability of DNA polymerase contribute to the genetic stability of an organism?
The proofreading ability is a critical error-correction mechanism that ensures genetic stability. As DNA polymerase adds nucleotides, it double-checks each new base against the template. If it detects a mismatch (an incorrect base pair), it pauses and its 3'→5' exonuclease activity removes the erroneous nucleotide. This process reduces the rate of replication errors by about a thousand-fold, preventing harmful mutations and ensuring that genetic information is passed on accurately to the next generation.
