RNA polymerases are enzyme complexes that synthesize RNA molecules using DNA as a template, in the process known as transcription. The RNAs created by transcription are either used as is (as ribosomal RNAs, transfer RNAs, or other types), or serve to guide the synthesis of a protein (as messenger RNAs). The word "polymerase" derives from "-ase," a suffix indicating an enzyme, and "polymer," meaning a large molecule composed of many similar parts, in this case the RNA nucleotides A, U, C, and G (abbreviations for adenine, uracil, cytosine, and guanine).

RNA polymerase use one side of the DNA double helix (blue) to assemble RNA nucleotides into an RNA transcript. The internal structure of the polymerase, shown in cut-away view, helps separate the DNA strands. A magnesium ion (Mg²⁺) helps catalyze the addition of RNA nucleotides into the growing chain. Adapted from .

Types of Polymerases

Prokaryotic organisms...