Nuclear Processing of mRNA

Once the bases are joined together in the nucleus to form mRNA, it is edited with a reduction in size. Through editing and processing, less than 10% of the original mRNA actually leaves the nucleus. Editing and processing are needed because immature RNA contains all those bases corresponding to the DNA introns. The removal of these introns is a cut-and-splice process whereby the intron is cut at its 5' end, pulled out of the way, and cut again at its 3' end. After this group of bases is excised, the bases corresponding to the DNA exons are joined. This cut-and-splice routine is continued until all the introns are removed and the exons joined. Some genes can give rise to multiple protein products since not all exons are necessarily retained in the mature mRNA. Some editing of the RNA also occurs with base substitutions made as appropriate. The mRNA is capped at the 5' end in a process that adds a guanine base and some methyl groups. Finally, a 3'-terminal poly A tail is added and the mature mRNA is ready to leave the nucleus and move to the cytoplasm for translation. The nucleotides that have been removed during editing and processing are either reused or degraded. Some mRNA is totally degraded, never leaving the nuclear compartment. This serves to control the amount of mRNA. Regulation of the amount of mRNA that leaves the nucleus is a key step in metabolic control.

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