Editing Primary transcript (section)

===Alternative splicing===
{{Main|Alternative splicing}}
Eukaryotic pre-mRNAs have their introns spliced out by [[spliceosome]]s made up of [[snRNP|small nuclear ribonucleoproteins]].<ref>{{cite book | vauthors = Weaver RF | date = 2005 | title = Molecular Biology | pages = 432–448 | publisher = McGraw-Hill| location = New York, NY| isbn = 0-07-284611-9}}</ref><ref>{{cite journal | vauthors = Wahl MC, Will CL, Lührmann R | title = The spliceosome: design principles of a dynamic RNP machine | journal = Cell | volume = 136 | issue = 4 | pages = 701–18 | date = February 2009 | pmid = 19239890 | doi = 10.1016/j.cell.2009.02.009 | hdl = 11858/00-001M-0000-000F-9EAB-8 | s2cid = 21330280 | hdl-access = free }}</ref>

In complex eukaryotic cells, one primary transcript is able to prepare large amounts of mature mRNAs due to alternative splicing. Alternative splicing is regulated so that each mature mRNA may encode a multiplicity of proteins. [[File:Alternativ splicing.png|thumb|Alternative splicing of the primary transcript]] The effect of alternative splicing in gene expression can be seen in complex eukaryotes which have a fixed number of genes in their genome yet produce much larger numbers of different gene products.<ref name="Cooper GM"/> Most eukaryotic pre-mRNA transcripts contain multiple introns and exons. The various possible combinations of 5' and 3' splice sites in a pre-mRNA can lead to different excision and combination of exons while the introns are eliminated from the mature mRNA. Thus, various kinds of mature mRNAs are generated.<ref name="Cooper GM"/> Alternative splicing takes place in a large protein complex called the [[spliceosome]]. Alternative splicing is crucial for tissue-specific and developmental regulation in gene expression.<ref name="Cooper GM"/> Alternative splicing can be affected by various factors, including mutations such as [[chromosomal translocation]].

In prokaryotes, splicing is done by [[autocatalytic]] cleavage or by endolytic cleavage. Autocatalytic cleavages, in which no proteins are involved, are usually reserved for sections that code for rRNA, whereas endolytic cleavage corresponds to tRNA precursors.