In molecular biology, RNA polymerase (abbreviated RNAP or RNApol, and officially RNA polymerase (dependent) directed by DNA) is an enzyme that synthesizes RNA from a DNA template.
Using the enzyme helicase, RNAP locally opens double-stranded DNA so that one strand of the exposed nucleotides can be used as a template for RNA synthesis, a process called transcription. A transcription factor and its associated transcription mediator complex must bind to a DNA-binding site called the promoter region before RNAP can initiate DNA unwinding at that position. RNAP not only initiates RNA transcription, but also guides nucleotides into position, facilitates binding and elongation, has intrinsic correction and replacement capabilities, and termination recognition capabilities. In eukaryotes, RNAP can build chains of up to 2.4 million nucleotides.
RNAP produces RNA that, functionally, is for the coding of proteins, that is to say, messenger RNA (mRNA); or non-coding (so-called "RNA genes"). There are at least four functional types of RNA genes:
Transfer RNA (tRNA): transfers specific amino acids to growing polypeptide chains at the ribosomal site of protein synthesis during translation;
Ribosomal RNA (rRNA): it is incorporated into ribosomes;
micro RNA (miRNA): regulates genetic activity; and,
Catalytic RNA (ribozyme) - works as an enzymatically active RNA molecule.
RNA polymerase is essential for life and is found in all living organisms and in many viruses. Depending on the organism, an RNA polymerase can be a protein complex (multi-subunit RNAP) or only consist of one subunit (single subunit RNAP, ssRNAP), each of which represents an independent lineage. The former is found in bacteria, archaea, and eukaryotes alike, and they share a similar core structure and mechanism.  The latter is found in phages, as well as chloroplasts and eukaryotic mitochondria, and is related to modern DNA polymerases.  Eukaryotic and archeal RNAPs have more subunits than bacterial ones and are controlled differently.
Bacteria and archaea only have one RNA polymerase. Eukaryotes have multiple types of nuclear RNAPs, each responsible for the synthesis of a distinct subset of RNA:
RNA polymerase I synthesizes a 45S pre-RNA (35S in yeast), which matures and will form the major RNA sections of the ribosome.
RNA polymerase II synthesizes mRNA precursors and most RNA and microRNA.
RNA polymerase III synthesizes tRNA, 5S rRNA, and other small RNAs found in the nucleus and cytosol.
RNA polymerase IV and V found in plants are less understood; make siRNA. In addition to ssRNAPs, chloroplasts also encode and use a bacterium-like RNAP.