What is DNA Polymerase?
There are several enzymes which help in synthesising and functioning of the DNA. One such enzyme is DNA polymerase.
Generally, it is a group of enzymes which are required for the synthesis of DNA. This was first observed by Arthur Kornberg in the E. coli during the process of purification.
It is generally a single polypeptide chain which is now commonly called as DNA-Polymerase I.
Scientists have recently found the presence of five enzymes of DNA polymerase in E-coli.
Features of DNA Polymerase
DNA polymerase is defined as the group of catalysing enzymes which are involved in the synthesis of DNA by the process of replication which takes place during cell division.
The main function of DNA polymerase is to duplicate the DNA content which is present in the cell during the process of cell division, by adding the nucleotides to the growing DNA strand.
The nucleotides present are of 3’-OH group.
Function of DNA Polymerase
The important function of DNA polymerase is to synthesise DNA by the process of replication. The process of DNA polymerase is to is to maintain and transfer the genetic information from present to future generation.
DNA polymerase works only in pairs which replicates the strands of DNA in tandem. It also adds deoxyribonucleotides in the 3’-OH group in the growing strand of DNA, which grows in the direction of 5’ to 3’ end in the activity of polymerisation. Where Adenine pairs up with thymine, Guanine pairs up with cytosine.
However, DNA polymerase does not initiate the replication process. To initiate this process, they must have their primer which have to be added to the nucleotides.
DNA polymerase III acts as a main enzyme which is responsible for replication in the process of prokaryotes.
DNA polymerase δ helps in replication processes in eukaryotes.
DNA polymerase I helps in removing the RNA primer by 5’ to 3’ exonuclease activity which replaces the primer by polymerase activity in the lagging strands.
The replication process is one of the important tasks to maintain the integrated of the genomes. Apart from considering the replication errors DNA repair also helps in correcting the errors in the genome during the damages in the DNA. DNA repair mechanism involves various processes.
DNA replication does not occur in a perfect manner it occurs with an error after every 104 to 105 nucleotides added. The other very important step to be followed is to remove the incorrect nucleotide sequences or mismatched nucleotide sequences for a newly synthesized strand which is very important for the proteins to get activated.
Sometimes the mismatching of pairs leas to the formation of cancer. DNA polymerases helps in removing the incorrect and mis match pairs by inducing the exonuclease activity which is known as proof reading.
These DNA polymerases are also involved in the post replication process of DNA repair and it is also involved in translesion synthesis by which DNA polymerase copies the pair that are not suited and also blocks the progression of DNA replication.
Types & Functions of DNA Polymerase in Prokaryotes
There are about five DNA polymerases which are observed in the E. coli, where each of the polymerases differ in their structure and functions and also in the rate of polymerisation and processivity.
DNA Polymerase I
DNA polymerase I is coded by the polA gene which has a single polypeptide and plays an important role in recombination and repairing the DNA strands.
It consists of 5’ to 3’ end and also the 3’ to 5’ exonuclease activity.
It also removes the RNA primer from lagging strand by 5’ to 3’ end exonuclease activity which fills the gap.
DNA Polymerase II
This is generally coded by gene named poIB. It consists of about 7 subunits and plays a vital role in repairing the DNA and also in backing up the DNA polymerase III.
It also helps in proofreading 3’ to 5’ exonuclease activity.
DNA Polymerase III
The DNA polymerase III that is present in E. coli plays an important role in replication of DNA. It is coded by the gene named poIC.
DNA polymerase III plays a much important role in polymerisation and in processing activities.
It also proofreads 4’ to 5’ end exonuclease activity. It is made up of thirteen sub units which also comprises nine different sub units.
It contains two core domains that are made up of α. ϵ . ϴ sub units which is attached to the γ – complex this is also called as clamp loading complex, made up of five subunits namely T2Yδδ’.
Additional subunits are X and ψwhich attached additionally to the clamp loading complex. Β sub units are made up two clamps with a dimer in each of the complexes, which helps in increasing the activity of DNA POLYMERASE III.
DNA Polymerase IV
This enzyme is coded by the gene dinB. DNA polymerase IV plays an important role in repairing DNA in response to the SOS. When the DNA replication is forked in the replication fork.
The DNA polymerases II, IV and V are translesion polymerase.
DNA Polymerase V
This enzyme is mainly involved in the synthesis of translesion during DNA repair and SOS response.
DNA polymerase V is made up of UmuC monomer and a UmuD dimer.
Types & Functions of DNA Polymerase in Eukaryotes
As prokaryotic cells eukaryotic cells also contains DNA polymerases which are present in many types and also has many specific functions auch as replication processes in mitochondria and in the nucleus. In the nuclear DNA replication is performed by DNA polymerase δ and α.
There are about fifteen DNA polymerases identified in humans till now.
DNA Polymerase δ
This is considered as one of the important enzymes in the replication of DNA in eukaryotes. It also has 3’ to 5’ exonuclease activity for proof reading.
DNA Polymerase α
The important function of this enzyme is to synthesize primers It forms the primer for okazagi fragments that are extended by DNA polymerase δ.
DNA Polymerase ϵ
It plays an important role in repairing the DNA. It also removes the primers of okazagi fragments in the lagging strand.
DNA Polymerase γ
It is important in the replication of mitochondrial DNA.