IntronEdit
Intron is a non-coding sequence within a gene that is transcribed into precursor RNA but is removed during RNA processing, yielding a mature messenger RNA composed of expressed sequences. Introns are a defining feature of many eukaryotic genes, shaping the architecture of transcription and post-transcriptional regulation. They were revealed in the late 20th century through work on adenoviruses and other eukaryotic systems, reshaping notions of how genes are organized and how information from DNA is converted into functional proteins gene transcription RNA splicing.
Most introns occur in the nuclear genomes of eukaryotes, and their presence varies widely across lineages. In vertebrates, plants, and many fungi, introns can be dozens to thousands of base pairs long, and the total intronic content can represent a large fraction of a gene’s length. In contrast, many prokaryotes have few or no introns, reflecting different evolutionary pressures on genome organization. There are also self-splicing introns in organellar genomes (group I and group II introns) that can remove themselves without the help of the spliceosome, illustrating that RNA molecules themselves can act as catalysts in genome editing processes Group I intron Group II intron ribonucleic acid ribozymes.
Structure and classification
Introns are elements within a gene that separate exons—the coding portions that remain in the final transcript. They are removed through a process known as splicing, typically occurring co-transcriptionally or soon after transcription begins. Canonical nuclear introns usually begin with a 5' splice site characterized by a GT dinucleotide and end at a 3' splice site characterized by an AG dinucleotide, with a short conserved region near these junctions called the branch point involved in forming a lariat intermediate during splicing. The entire process relies on a complex RNA-protein machine called the spliceosome, composed of small nuclear RNAs (snRNAs) and numerous proteins that recognize splice sites and catalyze the cutting and rejoining steps spliceosome 5' splice site 3' splice site RNA splicing.
Introns are categorized primarily by their splicing mechanism. The vast majority of nuclear introns in multicellular eukaryotes are spliceosomal introns, removed by the spliceosome. In contrast, self-splicing introns (group I and group II) depend on RNA folding and catalytic activity to excise themselves; group II introns, for example, are of particular interest because they are related to the catalytic cores of the spliceosome and share mechanistic features with it. The study of these introns provides insight into RNA catalysis, RNA structure, and the evolution of RNA-based processing systems Group II intron Group I intron ribonucleic acid ribozyme.
Length and composition of introns vary considerably. Some introns are short, a few dozen bases, while others span tens of kilobases, particularly in large genes. Introns often contain regulatory motifs, noncoding RNAs, and sequences that influence the efficiency and accuracy of splicing. Repetitive elements within introns can contribute to genome size and may affect chromatin structure and transcriptional dynamics, illustrating that introns are not simply passive spacers but active genomic features exon regulatory elements noncoding RNA.
Mechanism of splicing and regulation
Splicing—the removal of introns and joining of exons—occurs through tightly coordinated steps guided by the spliceosome. The process begins with recognition of the 5' splice site, branch point, and 3' splice site, followed by two transesterification reactions that release a lariat-shaped intron and join the flanking exons to produce mature mRNA. The precision of splice-site recognition is aided by auxiliary sequences known as exonic and intronic splicing enhancers and silencers, and by various splicing factors that regulate tissue- and development-specific patterns of mRNA production. Through these mechanisms, a single gene can produce multiple mRNA variants via alternative splicing, increasing proteomic diversity without expanding the genome spliceosome RNA splicing alternative splicing exon intron retention.
Alternative splicing is pervasive in many organisms and contributes to tissue specificity, developmental regulation, and adaptive responses. It allows different cell types to express distinct protein isoforms from the same gene, and it can also influence mRNA stability, subcellular localization, and translation efficiency. The balance of splicing activators and repressors, along with sequence elements in both introns and exons, determines which isoforms are produced in a given context. In some cases, misregulation of splicing is linked to disease, highlighting the clinical importance of understanding intron architecture and splicing control alternative splicing regulatory elements spliceosome.
Evolution, function, and controversy
The presence and abundance of introns have prompted extensive discussions about genome evolution. The dominant models—often summarized as intron-early and intron-late—offer different explanations for how introns originated and diversified. Some evidence supports ancient intron inroads into genes, while other data point to lineage-specific gains or losses. Regardless of the origin, introns appear to contribute to genetic innovation by enabling exon shuffling, modular domain rearrangement, and the storage of regulatory information. Introns also house regulatory sequences and noncoding RNAs, which can influence gene expression profiles in response to developmental cues or environmental signals exon shuffling regulatory elements noncoding RNA.
From a functional standpoint, introns help modulate transcriptional timing, mRNA export from the nucleus, and quality control of transcripts. They may buffer against deleterious mutations in coding regions and provide a substrate for evolutionary experimentation by permitting alternative splicing. Because splicing is essential for many genes, mutations at splice sites or in splicing factors can lead to disease, including congenital disorders and certain cancers. Therapeutic approaches that target splicing—such as antisense oligonucleotides designed to modulate splicing—illustrate how deep knowledge of intron structure and processing can inform medical innovation splice site mutation antisense therapy RNA sequencing.
Research into introns continues to reveal how genome architecture shapes biology. High-throughput sequencing technologies and computational models are mapping splicing variants across tissues and species, helping to connect intron structure with function and disease risk. The study of introns thus sits at the crossroads of molecular biology, evolution, and medicine, linking fundamental gene organization to the diversity of life and its health outcomes RNA sequencing genome transcriptomics.