POLYMERASE OR REPLICASE ENZYMES: A polymerase enzyme catalyses the formation of a polymer and cellular polymerase enzymes of genetic interest are those that bring about the synthesis of one polynucleotide chain that is copy of another. A polymerase enzyme is called replicase enzyme when the copy of the polynucleotide chain so produced is inherited by daughter cells or viruses, that is, when the enzyme brings about chromosomal replication.
PROKARYOTIC DNA POLYMERASES: Three different DNA polymerases are known in E.Coli and other prokaryotes, of which DNA polymerase I and II are meant for DNA repair and DNA polymerase III is meant for actual DNA replication.
1. DNA polymerase I. This enzyme was isolated around 1960 by Arthur Kornberg and was the first enzyme suggested to be involved in DNA replication. It is also called Kornberg enzyme. DNA polymerase I enzyme is now ...view middle of the document...
2. DNA polymerase II. This enzyme resembles DNA polymerase I in it’s activity, but is a DNA repair enzyme. It brings about the growth in 5’ 3’ direction, using free 3’-OH groups.
3. DNA polymerase III. Dna polymerase III or POLL III enzyme plays an essential role in DNA replication. It is a multimeric enzyme or holoenzyme having ten subunits such as alpha(), beta(), epsilon(), theta(), tau(), gamma(), delta(), delta dash(), chi(), and psi(). All these ten subunits are needed for DNa replication in-vitro; however, all havng different functions. For example, a subunit has 5’3’ exonuclease proof reading or editing activity. The core enzyme comprises of three subunits α,β, and θ. Remaining seven subunits increase progressivity (progressivity means rapidity and efficiency with which a DNA polymerase extends growing chain.)
EUKARYOTIC DNA POLYMERASES: Eukaryotes (yeast, rat liver, human tumor cells) are found to contain the following five types of DNA polymerases.
1. DNA polymerase α (alpha). This relatively high molecular weight enzyme is also called cytoplasmic polymerase or large polymerase. It is found in both nucleus and cytoplasm.
2. DNA polymerase β (beta). This enzyme is also called nuclear polymerase or small polymerase and is found only in vertebrates.
3. DNA polymerase ϒ (gamma). This enzyme is called mitochondrial enzyme and is encoded in the nucleus.
4. DNA polymerase δ (delta). This enzyme is found in mammalian cells and is PCNA (proliferating cell nuclear antigen) dependant for DNA synthesis processivity.
5. DNA polymerase ε(epsilon). It was previously known as DNA polymerase δ II. This enzyme is PCNA independent and occurs in mammalian HeLa cells and budding yeast.
The large DNA polymerase α id the predominant DNA polymerase enzyme in eukaryotic cells and was believed for long time to be the only enzyme involved in DNA replication. But now one more polymerase enzyme namely, DNA polymerase δ is found to be involved in DNA replication.