The main function of DNA polymerase is to make DNA from nucleotides, the building blocks of DNA.
There are several forms of DNA polymerase that play a role in DNA replication and they usually work in pairs to copy one molecule of double-stranded DNA into two new double stranded DNA molecules. This process is called semi-conservative replication. Every time a cell divides, DNA polymerase is required to help duplicate the cell’s DNA, so that a copy of the original DNA molecule can be passed to each of the daughter cells. In this way, genetic information is transmitted from generation to generation.
The DNA copies are created by the pairing of nucleotides to bases present on each strand of the original DNA molecule. This pairing always occurs in specific combinations, with cytosine and guanine always forming a pair and thymine and adenine always forming a pair.
The function of DNA polymerase is not quite perfect, with the enzyme making about one mistake for every billion base pairs copied. As well as creating new DNA copies, DNA polymerase is responsible for “proofreading” the strand so that misplaced base pairs can be corrected. This preserves the integrity of the orignal DNA strand that is passed onto the daughter cells.
Role of DNA polymerase in DNA replication
Before replication can take place, an enzyme called helicase unwinds the DNA molecule from its tightly woven form. The double stranded DNA is opened up or “unzipped” to give two single strands of DNA that can be used as templates for replication.
DNA polymerase then adds new, free nucleotides to the 3’ end of the newly-forming strand, elongating it in a 5’ to 3’ direction. However, DNA polymerase cannot begin the formation of this new chain on its own and can only add nucleotides to a pre-existing 3'-OH group. A primer is therefore needed, at which nucleotides can be added. Primers are usually composed of RNA and DNA bases and the first two bases are always RNA. These primers are made by another enzyme called primase.
Once the copying is done, DNA polymerase corrects the mistakes in the newly-synthesized DNA by proofreading the newly-made strands. When an incorrect coupling is recognized, the DNA polymerase reverses its direction by one base pair of DNA. The incorrect base pair is then excised and DNA polymerase tries to re-insert the correct nucleotide before continuing forwards.