DNA polymerases

DNA polymerases (or deoxynucleotidyl transferase) are the enzymes responsible for the polymerization of nucleotides during DNA replication. They are dependent on DNA, that is to say, they need a DNA template to produce the newly synthesized strand, and for this they read the template strand of 3 'to 5' to synthesize DNA in the 5’ to 3'.

Prokaryotic DNA polymerases are 3 types (I, II and III) and eukaryotic DNA polymerases are 5 types (α, β, δ, ε and γ).

DNA polymerases require a number of conditions of activities:

- The four deoxyribonucleotides 5' triphosphate (dATP, dTTP, dCTP and dGTP) in equimolar amounts.

- Magnesiums ions (Mg2 +) which stabilize DNA and proteins.

- A template DNA (single or double stranded).

- A DNA or RNA primer with a free 3' OH end.

Activities of DNA polymerases:

During the formation of a phosphodiester bond between deoxyribonucleotides 5' triphosphate and the strand being elongation, there is hydrolyzed to the function triphosphate and formation of pyrophosphate (PPi).

DNA polymerases have very specific activities:

- A polymerase activity 5' to 3' which is their main activity.

- Exo-nuclease activity correspondes in the degradation of one end of the newly synthesized strand of the DNA during replication and which can be of 2 types:

        -From 3' to 5', which corresponds to the degradation from the 3' OH end. The exonuclease activity 3' to 5' allows so-called proofreading, which corresponds to the correction of a mismatch base by breaking the phosphodiester bond and replacing the mismatched nucleotide.

        - From 5' to 3', which corresponds to the degradation from the 5 'phosphate end, at the junction of the DNA segments of the lagging strand synthesized.