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An intron refers to any nucleotide sequence within a gene that is removed by RNA splicing during maturation of the final RNA product.
It refers to both the DNA sequence within a gene and the corresponding sequence in RNA transcripts.
Sequences that are joined together in the final mature RNA after RNA splicing are exons.
They are found in a wide range of genes, including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA).
The frequency of introns within different genomes is observed to vary widely across the spectrum of biological organisms, but human protein-coding genes almost always contain multiple introns,
At least four distinct classes of introns have been identified:
Introns in nuclear protein-coding genes that are removed by spliceosomes (spliceosomal introns)
Introns in nuclear and archaeal transfer RNA genes that are removed by proteins (tRNA introns)
Self-splicing group I introns that are removed by RNA catalysis
Self-splicing group II introns that are removed by RNA catalysis
Introns do not encode protein products, they are integral to gene expression regulation.
Some introns encode functional RNAs through further processing after splicing to generate noncoding RNA molecules.
Some introns play essential roles in a wide range of gene expression regulatory functions such as Nonsense-mediated decay and mRNA export.
The human genome contains an average of 8.4 introns/gene.
Biological factors also influence which genes in a genome loose or accumulate introns.
Alternative splicing of introns within a gene acts to introduce greater variability of protein sequences translated from a single gene, so multiple related proteins can be generated from a single gene..