Editing Gene expression (section)

===Transcription===
[[File:simple transcription elongation1.svg|thumb|400px|alt=RNA polymerase moving along a stretch of DNA, leaving behind newly synthetized strand of RNA.|The process of transcription is carried out by RNA polymerase (RNAP), which uses DNA (black) as a template and produces RNA (blue).]]
{{main|Transcription (biology)}}
The production of a RNA copy from a DNA strand is called [[Transcription (biology)|transcription]], and is performed by [[RNA polymerase]]s, which add one [[ribonucleotide]] at a time to a growing RNA strand as per the [[Complementarity (molecular biology)|complementarity]] law of the nucleotide bases. This RNA is [[Complementarity (molecular biology)|complementary]] to the template 3′ → 5′ DNA strand,<ref name="Brueckner2009">{{cite journal | vauthors = Brueckner F, Armache KJ, Cheung A, Damsma GE, Kettenberger H, Lehmann E, Sydow J, Cramer P | title = Structure-function studies of the RNA polymerase II elongation complex | journal = Acta Crystallographica. Section D, Biological Crystallography | volume = 65 | issue = Pt 2 | pages = 112–120 | date = February 2009 | pmid = 19171965 | pmc = 2631633 | doi = 10.1107/S0907444908039875 | bibcode = 2009AcCrD..65..112B }}</ref> with the exception that [[thymine]]s (T) are replaced with [[uracil]]s (U) in the RNA and possible errors.

[[Bacterial transcription|In bacteria, transcription]] is carried out by a single type of RNA polymerase, which needs to bind a DNA sequence called a [[Pribnow box]] with the help of the [[sigma factor]] protein (σ factor) to start transcription. In eukaryotes, transcription is performed in the nucleus by three types of RNA polymerases, each of which needs a special DNA sequence called the [[Promoter (biology)|promoter]] and a set of DNA-binding proteins—[[transcription factors]]—to initiate the process (see regulation of transcription below). [[RNA polymerase I]] is responsible for transcription of ribosomal RNA (rRNA) genes. [[RNA polymerase II]] (Pol II) transcribes all protein-coding genes but also some non-coding RNAs (''e.g.'', snRNAs, [[snoRNA]]s or [[long non-coding RNA]]s). [[RNA polymerase III]] transcribes [[5S rRNA]], transfer RNA (tRNA) genes, and some small non-coding RNAs (''e.g.'', [[7SK RNA|7SK]]). Transcription ends when the polymerase encounters a sequence called the [[Terminator (genetics)|terminator]].