Overview of Post-Transcriptional RNA Splicing
Prokaryotic DNA transcription produces messenger RNA, which is necessary for transfer from the cell nucleus to the cytoplasm where translation occurs. In contrast, eukaryotic DNA transcription takes place in a cell's nucleus and produces what is called a primary RNA transcript or pre-messenger RNA. Before eukaryotic products of transcription can be moved into the cytoplasm, they must undergo modifications that allow them to become mature messenger RNA. Splicing is the name given to the reaction that removes unnecessary segments of the primary RNA transcript, called introns. The removal of the introns produces mRNA (see the figure, below). Messenger RNA contains only exons, those portions of the primary RNA transcript that will be translated into a protein. Unlike the sequence of an exon, intron sequences are unimportant. Only small portions of an intron sequence are preserved. These portions, located near the end of each intron, serve to identify a sequence as an intron, identifying the sequence for removal.