Editing Cell nucleus (section)

===Processing of pre-mRNA===
{{Main|Post-transcriptional modification}}
Newly synthesized mRNA molecules are known as [[primary transcript]]s or pre-mRNA. They must undergo [[post-transcriptional modification]] in the nucleus before being exported to the cytoplasm; mRNA that appears in the cytoplasm without these modifications is degraded rather than used for protein translation. The three main modifications are [[5' cap]]ping, 3' [[polyadenylation]], and [[RNA splicing]]. While in the nucleus, pre-mRNA is associated with a variety of proteins in complexes known as [[heterogeneous ribonucleoprotein particle]]s (hnRNPs). Addition of the 5' cap occurs co-transcriptionally and is the first step in post-transcriptional modification. The 3' poly-[[adenine]] tail is only added after transcription is complete.<ref name = "Lodish" />{{rp|509–18}}

RNA splicing, carried out by a complex called the [[spliceosome]], is the process by which introns, or regions of DNA that do not code for protein, are removed from the pre-mRNA and the remaining [[exon]]s connected to re-form a single continuous molecule. This process normally occurs after 5' capping and 3' polyadenylation but can begin before synthesis is complete in transcripts with many exons.<ref name="Lodish" />{{rp|494}} Many pre-mRNAs can be spliced in multiple ways to produce different mature mRNAs that encode different [[primary structure|protein sequences]]. This process is known as [[alternative splicing]], and allows production of a large variety of proteins from a limited amount of DNA.<ref name=Black>{{cite journal | vauthors = Black DL | title = Mechanisms of alternative pre-messenger RNA splicing | journal = Annual Review of Biochemistry | volume = 72 | issue = 1 | pages = 291–336 | year = 2003 | pmid = 12626338 | doi = 10.1146/annurev.biochem.72.121801.161720 | s2cid = 23576288 | url = https://cloudfront.escholarship.org/dist/prd/content/qt2hg605wm/qt2hg605wm.pdf  | department = Review }}</ref>
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