RNA polymerase enzymes are essential to life and are found in all organisms and many . In chemical terms, RNAP is a nucleotidyl transferase that polymerizes ribonucleotides at the 3′ end of an RNA transcript. RNAP can initiate transcription at specific DNA sequences known as promoters. It then produces an RNA chain, which is complementary to the template DNA strand. The process of adding nucleotides to the RNA strand is known as elongation; in eukaryotes, RNAP can build chains as long as 2.4 million nucleotides (the full length of the dystrophin gene). RNAP will preferentially release its RNA transcript at specific DNA sequences encoded at the end of genes, which are known as terminators. Products of RNAP include: Messenger RNA (mRNA)—template for the synthesis of by ribosomes. Non-coding RNA or “RNA genes”—a broad class of genes that encode RNA that is not translated into protein. The most prominent examples of RNA genes are transfer RNA (tRNA) and ribosomal RNA (rRNA), both of which are involved in the process of translation. Transfer RNA (tRNA)—transfers specific amino acids to growing polypeptide chains at the ribosomal site of protein synthesis during translation Ribosomal RNA (rRNA)—a component of ribosomes Micro RNA—regulates gene activity