mRNA 3'-end processing Edited: Joshi-Tope, G, 0000-00-00 00:00:00 GENE ONTOLOGYGO:0031124 Pubmed10357856 Pubmed10371034 Pubmed11909521 Pubmed2408761 Reactome Database ID Release 4372187 Reactome, http://www.reactome.org ReactomeREACT_1849 The 3' ends of eukaryotic mRNAs are generated by posttranscriptional processing of an extended primary transcript. For almost all RNAs, 3'-end processing consists of two steps: (i) the mRNA is first cleaved at a particular phosphodiester bond downstream of the coding sequence, (ii) the upstream fragment then receives a poly(A) tail of approximately 250 adenylate residues, whereas the downstream fragment is degraded. The two partial reactions are coupled so that reaction intermediates are usually undetectable. While 3' processing can be studied as an isolated event <i>in vitro</i>, it appears to be connected to transcription, splicing, and transcription termination <i>in vivo</i>.<p>The only known exception to the rule of cleavage followed by polyadenylation are the major histone mRNAs, which are cleaved but not polyadenylated. mRNA polyadenylation Cleavage of mRNA at the 3'-end PathwayStep49 PathwayStep50