Transcription of protein encoding genes creates a primary transcript of RNA at the place where the gene was located. This transcript can be altered before being translated, this is particularly common in eukaryotes. The most common RNA processing is splicing to remove introns. Introns are RNA segments which are not found in the mature RNA, although they can function as precursors, e.g. for snoRNAs, which are RNAs that direct modification of nucleotides in other RNAs. Introns are common in eukaryotic genes but rare in prokaryotes.
RNA processing, also known as post-transcriptional modification, can start during transcription, as is the case for splicing, where the spliceosome removes introns from newly formed RNA.
Extensive RNA processing may be an evolutionary advantage made possible by the nucleus of eukaryotes. In prokaryotes transcription and translation happen together whilst in eukaryotes the nuclear membrane separates the two processes giving time for RNA processing to occur.
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