To discriminate the newly incorporated nucleotides determines the definition of polymerases

 All polymerases including DNA dependent DNA polymerase, DNA dependent RNA polymerase, RNA dependent RNA polymerase and RNA dependent RNA polymerase share a common polymerase domain in their structures. One of the most important functions of these enzymes is to distinguish between deoxyribonucleotides (dNTP) and ribonucleotide (rNTP), which principally determine the definition of polymerases; DNA polymerase or RNA polymerase. Several reports have shown that an amino acid residue located within the polymerase catalytic domain plays a critical role for selecting the newly incorporated nucleotides by recognizing their 2'- state. If the amino acid residue in an enzyme can bind to 2'-OH, not to 2'-H, this enzyme is defined as an RNA polymerase because rNTP has 2'-OH, not 2'-H. Thus, the mechanism, which discriminates the incorporated nucleotide to be dNTP or rNTP, determines the fundamental enzymatic property of polymerase.