Intron

An intron is a segment of a gene transcribed into precursor RNA and removed during RNA processing, usually by splicing. It separates exons in many genes.

Spliceosome: A ribonucleoprotein complex that removes introns from precursor messenger RNA. It recognizes splice sites and catalyzes the removal of many nuclear introns.
Exon: A gene segment retained in a processed RNA, although not every exon encodes protein. Exons are joined together after intervening introns are removed.
Intron early hypothesis: The hypothesis that introns were abundant in ancestral genes before being lost from many lineages. It proposes an ancient origin for introns, unlike models emphasizing later gains.
Intron-mediated enhancement: An increase in gene expression associated with the presence of an intron in a transcribed sequence. Some introns boost expression beyond their role in separating exons.
Exon shuffling: The evolutionary rearrangement of exons into new combinations within genes. Introns can provide boundaries that facilitate recombination between exons.
Splice site: A sequence boundary in RNA or DNA that directs the joining or removal of RNA segments. Splice-site signals mark where an intron begins and ends.
Pre-mRNA: An initial RNA transcript that undergoes processing to produce mature messenger RNA. It contains introns before splicing removes them.
Intron late hypothesis: The hypothesis that introns arose after early genes had already evolved without them. It offers an alternative account of how introns entered eukaryotic genes.
Intronless gene: A gene that contains no introns within its transcribed region. Its expression and evolution can differ from those of intron-containing genes.
Nonsense-mediated decay: A cellular pathway that degrades messenger RNAs containing premature termination codons. Splicing patterns and exon–intron junctions can help mark transcripts for this surveillance pathway.

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