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DNA Polymerase - DNA polymerase enzyme is responsible for constructing new DNA strands. DNA polymerase enzyme catalyses the addition or synthesis of new complementary strands using a template strand of DNA and all four deoxyribonucleotide triphosphate. There are three major DNA polymerase enzymes that will polymerize nucleotides into a growing strand of DNA in E.coli.

These enzymes are DNA polymerase I, II and III. DNA pol I and II are primarily used for DNA repair and for replicating a small length or segment of DNA. All the DNA polymerase that have been studied have two basic requirements:

1. A template DNA strand to copy.

2. A primer strand to which nucleotides can be added.

Likewise all DNA polymerase enzymes have two basic properties:

1. 5' -3' polymerization capacity

2. 5' -3' and exonuclease activity

The 5'-3' exonuclease activity of DNA pol I differs from the 3' -5' exonuclease used by both DNA pol I and pol III in two different/important ways. Firstly 5'-3' exonuclease can remove one nucleotide at a time from a region of DNA that is improperly base-paired.

The nucleotides it removes can be either ribonucleotides or deoxyribonucleotide. Secondly,5' -3 ' exonuclease can also remove groups of altered nucleotides in 5 ' -3 ' direction, removing from one to ten nucleotides at a time. This ability is important in the repair of damaged DNA.

A typical bacterial cell contains approximately 300 to 400 molecules of DNA pol I per cell and only about 40 copies of DNA polymerase II and 10 copies of DNA pol III. Eukaryotes have 9 types of DNA polymerases named α, β, γ, δ, ε, ξ, η, θ and τ. DNA polymerase δ seems to be the major replicating enzyme in eukaryotes.

 
   
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