<P> RNA polymerase (ribonucleic acid polymerase), both abbreviated RNAP or RNApol, official name DNA - directed RNA polymerase, is a member of a family of enzymes that are essential to life: they are found in all organisms (species) and many viruses . RNAP locally opens the double - stranded DNA (usually about four turns of the double helix) so that one strand of the exposed nucleotides can be used as a template for the synthesis of RNA, a process called transcription . A transcription factor and its associated transcription mediator complex must be attached to a DNA binding site called a promoter region before RNAP can initiate the DNA unwinding at that position . RNAP has intrinsic helicase activity, therefore no separate enzyme is needed to unwind the DNA (in contrast to DNA polymerase). RNAP not only initiates RNA transcription, it also guides the nucleotides into position, facilitates attachment and elongation, has intrinsic proofreading and replacement capabilities, and termination recognition capability . In eukaryotes, RNAP can build chains as long as 2.4 million nucleotides . </P> <P> RNAP produces RNA that functionally is either coding (for protein) (messenger RNA) (mRNA); or non-coding: so - called "RNA genes". At least four functional types of RNA genes exist: Transfer RNA (tRNA) - transfers specific amino acids to growing polypeptide chains at the ribosomal site of protein synthesis during translation; Ribosomal RNA (rRNA) - is a component of ribosomes; micro RNA (miRNA) - regulates gene activity; and catalytic RNA (Ribozyme)) - an enzymatically active RNA molecule . </P> <P> Eukaryotes have multiple types of nuclear RNAP, each responsible for synthesis of a distinct subset of RNA . All are structurally and mechanistically related to each other and to bacterial RNAP . RNA polymerase I synthesizes a pre-rRNA 45S (35S in yeast). which matures and will form the major RNA sections of the ribosome . RNA polymerase II synthesizes precursors of mRNAs and most snRNA and microRNAs. RNA polymerase III synthesizes tRNAs, rRNA 5S and other small RNAs found in the nucleus and cytosol . RNA polymerase IV synthesizes siRNA in plants . RNA polymerase V synthesizes RNAs involved in siRNA - directed heterochromatin formation in plants . Eukaryotic chloroplasts contain an RNAP very highly structurally and mechanistically similar to bacterial RNAP ("plastid - encoded polymerase"). They also contain a second, structurally and mechanistically unrelated, RNAP ("nucleus - encoded polymerase"; member of the "single - subunit RNAP" protein family). Eukaryotic mitochondria contain a structurally and mechanistically unrelated RNAP (member of the "single - subunit RNAP" protein family). </P> <P> Given that DNA and RNA polymerases both carry out template - dependent nucleotide polymerization, it might be expected that the two types of enzymes would be structurally related, but they are not . They seem to have arisen independently twice during the early evolution of cells: one lineage led to the modern DNA polymerases and reverse transcriptases, as well as to a few single - subunit RNA polymerases from viruses; the other lineage formed all of the modern cellular RNA polymerases . </P>

Where does rna polymerase initiate synthesis of mrna
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