DNA polymerase is the enzyme responsible for forming new copies of DNA, in the form of nucleic acid molecules. DNA replication is the cellular process involved in the synthesis of an exact copy of an existing DNA molecule. During DNA replication, DNA polymerase reads the existing/template DNA strand while synthesizing a new, complementary DNA strand to the template. It adds nucleotides to the 3’end of the growing strand, one nucleotide at a time. In addition, DNA polymerase is involved in proofreading the synthesized DNA.
Key Areas Covered
Key Terms: DNA Polymerase, DNA Replication, 3′ to 5′ Exonuclease Activity, Proofreading
What is DNA Polymerase
DNA polymerase is the enzyme responsible for DNA replication. It adds complementary nucleotides to the growing DNA strand, depending on the nucleotides in the template strand. Prokaryotes have DNA polymerases I to V. DNA polymerase I and III are responsible for 80% of DNA replication in prokaryotes. Eukaryotes have DNA polymerases α, β, λ, γ, σ, μ, δ, ε, η, ι, κ, ζ, θ, and Rev1. DNA polymerase is shown in figure 1.
Retroviruses such as RNA viruses use RNA-dependent DNA polymerase to synthesize DNA from an RNA template.
What is DNA Replication
DNA replication is a cellular process by which an exact copy of a particular DNA molecule is synthesized. It occurs during the S phase of interphase, prior to cell division. Generally, DNA is a double-stranded molecule and both its strands serve as templates for DNA replication. In addition, each of the newly synthesized DNA molecules consists of an old DNA strand. Therefore, DNA replication is a semi-conservative process. DNA replication is shown in figure 2.
DNA replication involves several enzymes and many proteins. Helicase, RNA primase, and DNA polymerase are some enzymes involved in replication. Transcription factors are the proteins involved in DNA replication.
What is the Role of DNA Polymerase in Replication
DNA polymerase performs several functions during replication. The main function of DNA polymerase is to synthesize a new DNA strand. Apart from this, DNA polymerase is also involved in correcting the errors of added nucleotides in a process known as proofreading. Proofreading helps to maintain the integrity of the double-stranded DNA.
- Synthesis of a new DNA strand – DNA polymerase adds complementary nucleotides to both leading and lagging strands in 3′ to 5′ direction. It requires an RNA primer for the initiation of the process, which is synthesized by RNA primase. DNA polymerase starts adding complementary nucleotides to the template strand from the 3’end of the primer.
- Double-checking the incoming nucleotide – Generally, the correct complementary nucleotide pairs with the new, incoming nucleotides in the growing chain. Before forming phosphodiester bond, DNA polymerase double-checks the paired nucleotide. The proofreading is shown in figure 3.
- Exonucleolytic proofreading – However, some incorrect nucleotides can be added to the growing chain. Immediately after the addition of such nucleotides, a separate catalytic subunit of DNA polymerase with 3′ to 5′ exonuclease activity digests the incorrectly paired nucleotide from the growing strand and resynthesizes the correct nucleotide.
DNA polymerase is the enzyme responsible for the synthesis of new DNA strand by using the existing DNA strand as a template. Other than that, DNA polymerase is also equipped with proofreading mechanisms to maintain the integrity of DNA.
1. Garcia-Diaz, Miguel, and Katarzyna Bebenek. “Multiple Functions of DNA Polymerases.” Critical Reviews in Plant Sciences, U.S. National Library of Medicine, Mar. 2007, Available here.
1. “DNA polymerase” By Yikrazuul – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “0323 DNA Replication” By OpenStax – (CC BY 4.0) via Commons Wikimedia
3. “DNA polymerase” By I, Madprime (CC BY-SA 3.0) via Commons Wikimedia