Enzyme of the Month February edition - DNA Polymerase

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We all know that little kids grow with time, we also know that a skin injury is healed when healthy skin is replaced. Do we know how little children grow up physically? Or do we know how new skin grows? The simple answer is that the cell replicates. In order to replicate, the genetic material needs to be copied. It is a complex process that involves various enzymes. The star of this process is the enzyme DNA polymerase. Before we dive into DNA polymerase, we will describe the process of DNA replication. 

DNA replication in eukaryotes (eukaryotes are described as organisms with cells consisting of a nuclear envelope) begins with an origin of replication where the DNA strand is unwounded. Now the DNA replication has a peculiar fact, it can only be synthesized in one direction which is the 3’hydroxl end to the 5’ phosphate end of the DNA. This enables the DNA strands to form a bond with other DNA strands. DNA is very particular about this rule and that is why the enzymes are designed in a way that suits the fancy of the unbending attitude of DNA. Enzymes, therefore, produce an RNA primer that provides a 3’hydroxyl end to the growing template strand of the DNA. The template or the old strand is used to synthesize a new strand by adding new bases in a complementary manner. Adenine is paired with Thymine, Guanine is paired with Cytosine. After the genetic material is copied, the DNA molecule is wounded. This process is very similar in the prokaryotes; however, the DNA polymerase enzymes used are different. 

DNA polymerase is divided into families of polymerase enzymes that carry out different functions. This blog focuses on A, B, and C families. The A family of the polymerase is involved with the repair and elongation of the DNA strands. Whereas the B family of the polymerase is important for checking base (Adenine is paired with Thymine and Guanine is paired with Cytosine) mismatch in the DNA strands. Finally, the C family is reserved for the bacterial DNA elongation. Blocking the function of the C family enzymes can produce an antibacterial effect as the DNA elongation is halted and the bacteria cannot grow anymore. Hence, preventing bacterial growth. 

With the process of DNA replication discussed, we can now begin looking at the enzyme DNA polymerase and its magnificent role in this super important process. We will first focus on the types of DNA polymerase in the prokaryotes. DNA polymerase I belong to the A family of the polymerase enzyme. Like the other A-family enzymes, this type of polymerase helps in strand elongation of the bacterial DNA. This enzyme adds bases on the 3’OH  end of the new strand. DNA polymerase II is useful for checking the base mismatch in the newly elongating strand. As this enzyme is involved in the base mismatch check, it belongs to family B of the polymerase enzymes. DNA polymerase III is a busy enzyme! It can add bases and can also check base mismatches! It is a part of the C family of enzymes.

Now we can move on with the DNA polymerases in the eukaryotes. The biologists are very particular with the naming process as you can tell. The DNA polymerases in the prokaryotes are identified with the Roman numerals whereas the eukaryotic DNA polymerases are identified with the Greek alphabets. The DNA Polymerase alpha initiates the DNA replication process by synthesizing an RNA primer. After the primer is synthesized, the DNA polymerase beta and epsilon, take over the show by synthesizing the lagging and the leading strand. As the DNA is anti-parallel (meaning the DNA is organized with one strand running from 5’ to 3’ end and the other 3’ to 5’ end), the template strand is elongated in a lagging and leading manner because of the fact that DNA bases are added in the 3’ end. Please refer to the image below)


Image Citation Link



Works Cited:

News-Medical. (2019, May). Eukaryotic DNA Polymerase Enzymes. News-Medical.net. https://www.news-medical.net/life-sciences/Eukaryotic-DNA-Polymerase-Enzymes.aspx

 

Knapp, S. (2020, May 24). DNA Polymerase. Biology Dictionary. https://biologydictionary.net/dna-polymerase/

 

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