The process in living cells in which the genetic information of DNA is transferred to a molecule of messenger RNA (mRNA) as the first step in protein synthesis (see also genetic code). Transcription takes place in the cell nucleus or (in prokaryotes) in the nuclear region. It involves the action of RNA polymerase enzyme in assembling the nucleotides necessary to form a complementary strand of mRNA from the DNA template (see also promoter), and (in eukaryote cells) the subsequent removal of the noncoding sequences from this primary transcript (pre-mRNA; see RNA processing) to form a functional mRNA molecule. The process begins with initiation, the binding of RNA polymerase to a sequence of bases ‘upstream’ of the coding sequence called the promoter, which orients the polymerase to the correct DNA strand—the template strand—and in the correct ‘downstream’, or 5′ to 3′, direction. Binding of the enzyme is assisted by transcription factors and additionally, in prokaryotes, by sigma factors. The two DNA strands unwind in the vicinity of the enzyme, and transcription begins at an initiation site in the promoter region. In the elongation phase, the chain of RNA nucleotides complementary to the base sequence of the template DNA is assembled by the polymerase from a pool of free ribonucleoside triphosphates (NTPs). The polymerase moves along the DNA template, adding nucleotides to the 3′ end of the growing RNA chain. Formation of the covalent bonds linking the nucleotides is driven by hydrolysis of two of the three phosphate groups attached to each incoming NTP. As elongation proceeds, the DNA double helix rewinds behind and unwinds in front of the polymerase, so that only about 10 base pairs are unwound at any time. Elongation continues until RNA polymerase reaches a termination sequence, where transcription stops and the pre-mRNA transcript is released (see terminator). In eukaryotes, the extent of DNA packaging in chromatin must be modified before a region of DNA can be transcribed. At sites of active transcription the normally supercoiled chromatin structure is decondensed and loops out, consisting of naked DNA or individual linked nucleosomes. The term ‘transcription’ is also applied to the assembly of single-stranded DNA from an RNA template by the enzyme reverse transcriptase. Compare translation.