DNA Polymerases
DNA Polymerases

DNA polymerase is an enzyme that assists in DNA replication. Such enzymes catalyze the polymerization of deoxyribonucleotides alongside a DNA strand, which they "read" and use as a template.The newly-polymerized molecule is complementary to the template strand and identical to the template's partner strand. DNA polymerase is considered to be a holoenzyme since it requires a magnesium ion as a co-factor to function properly. In the absence of the magnesium ion, it is referred to as an apoenzyme.

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Function of DNA Polymerase
Function of DNA Polymerase
DNA polymerase can add free nucleotides to only the 3’ end of the newly-forming strand. This results in elongation of the new strand in a 5'-3' direction. No known DNA polymerase is able to begin a new chain (de novo). DNA polymerase can add a nucleotide onto only a preexisting 3'-OH group, and, therefore, needs a primer at which it can add the first nucleotide. Primers consist of RNA and DNA bases with the first two bases always being RNA, and are synthesized by another enzyme called primase. An enzyme known as a helicase is required to unwind DNA from a double-strand structure to a single-strand structure to facilitate replication of each strand consistent with the semiconservative model of DNA replication.
Variation in DNA Polymerases Across Species
DNA polymerases have highly-conserved structure, which means that their overall catalytic subunits vary, on a whole, very little from species to species. Conserved structures usually indicate important, irreplicable functions of the cell, the maintenance of which provides evolutionary advantages. Some viruses also encode special DNA polymerases that may selectively replicate viral DNA through a variety of mechanisms. Retroviruses encode an unusual DNA polymerase called reverse transcriptase, which is an RNA-dependent DNA polymerase (RdDp). It polymerizes DNA from a template of RNA.
DNA Polymerase Families
DNA Polymerase Families
Based on sequence homology, DNA polymerases can be further subdivided into seven different families: A, B, C, D, X, Y, and RT. Family A polymerases contain both replicative and repair polymerases. Family B polymerases mostly contain replicative polymerases and include the major eukaryotic DNA polymerases. Family C polymerases are the primary bacterial chromosomal replicative enzymes. Family D polymerases are still not very well characterized. Family X contains the well-known eukaryotic polymerase pol â, as well as other eukaryotic polymerases such as pol ó, pol ë, pol ì, and terminal deoxynucleotidyl transferase (TdT). The Y-family polymerases differ from others in having a low fidelity on undamaged templates and in their ability to replicate through damaged DNA. Family RT the reverse transcriptase family contains examples from both retroviruses and eukaryotic polymerases.
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