text: RNA polymerase (RNAP or RNApol) is an enzyme responsible for making RNA from a DNA or RNA template. RNAP accomplishes this task by constructing RNA chains through a process termed transcription. In scientific terms, RNAP is a nucleotidyl transferase that polymerizes ribonucleotides at the 3' end of an RNA transcript. RNA polymerase enzymes are essential and are found in all organisms, cells, and many viruses.\nEukaryotes have several types of RNAP:\n\nRNA polymerase I synthesizes a pre-rRNA 45S, which matures into 28 S, 18S and 5,8S rRNAs which will form the major RNA sections of the ribosome. \n\nRNA polymerase II synthesizes precursors of mRNAs and most snRNA. This is the most studied type, and due to the high level of control required over transcription a range of transcription factors are required for its binding to promoters. For detail of RNA polymerase function please see RNA polymerase II. \n\nRNA polymerase III synthesizes tRNAs, rRNA 5S and other small RNAs found in the nucleus and cytosol. \n\nOther RNA polymerase types in mitochondria and chloroplasts.