Although introns are typically removed from pre-mRNA molecules after they are transcribed, these sequences may serve a regulatory function, be involved in alternative splicing, or may correspond to ancient genes that are no longer functional.
A significant number of eukaryotic genes have introns that have to be spliced out. Introns are rarer in bacterial genomes, but do occur occasionally and are also removed. These sequences can produce non-coding RNA, which serve regulatory functions. Alternative splicing of introns can also introduce variability in the mRNA molecules produced from a single gene, leading to the formation of multiple proteins from that gene. Alternatively, introns may be nonfunctional sequence remnants left over from the fusion of ancient genes during evolution. This is supported by the fact that separate exons often encode separate protein subunits or domains. For the most part, the sequences of introns can be mutated without ultimately affecting the protein product.
• Introns help create variation in the mRNA molecules produced from a gene and thus the resulting proteins.
• Non-coding RNA may get produced from introns.
• Introns may have once encoded proteins but these functions were lost over the course of evolution.
Introns: Regions of a pre-mRNA molecule that do not encode a protein sequence.
Non-coding RNA: An RNA molecule that does encode a protein, but may serve a regulatory function; snRNAs that make up the spliceosome are an example.
Alternative splicing: A process following transcription that can produce multiple mRNA molecules from a single gene.