Gene Anatomy: Exons, Introns, and Splice Variants

Eukaryotic genes are split: their protein-coding instructions (exons) are interrupted by non-coding stretches (introns) that are transcribed but then removed before the message is used. By choosing which exons to keep, a single gene can produce several distinct mRNAs and proteins through alternative splicing, making exon-intron anatomy a major source of biological complexity.

Definition

Exons are the segments of a gene retained in the mature RNA, introns are the intervening segments removed during RNA processing, and splice variants (isoforms) are the distinct mature transcripts produced when a gene's exons are joined in different combinations.

Scope

This topic covers the exon-intron architecture of genes, the splicing process that removes introns and joins exons, and alternative splicing as the mechanism that generates multiple transcript variants from one gene. It treats gene anatomy as reference and educational material; splicing disease mechanisms are described in general terms rather than as clinical guidance.

Core questions

• What distinguishes an exon from an intron?
• How does splicing remove introns and join exons accurately?
• How does alternative splicing produce multiple proteins from one gene?
• Why does disrupting a splice site change gene function?

Key concepts

• Exon
• Intron
• Split (interrupted) gene
• Pre-mRNA splicing and the spliceosome
• Splice donor and acceptor sites
• Alternative splicing
• Transcript isoform
• Constitutive versus alternative exons

Mechanisms

After transcription, the spliceosome recognizes sequences at intron boundaries — the splice donor (5') and acceptor (3') sites — and excises each intron, ligating the flanking exons into a continuous mature RNA. Because exon inclusion is regulated, a gene can assemble different combinations of its exons in different cells or conditions, yielding alternative splice variants from one locus; this dramatically expands the protein-coding capacity of the genome. The pattern of alternative splicing is itself evolutionarily variable and tissue-specific, contributing to differences between cell types and species.

Clinical relevance

Variants that fall in splice sites or that create or destroy splicing signals can change which exons are included and therefore alter or abolish a gene's product, which is why splice-affecting variants are an important class in variant interpretation. This topic describes the structural basis of such effects for reference and education and does not provide diagnostic or treatment guidance.

Epidemiology

Alternative splicing is pervasive: the large majority of human multi-exon genes produce more than one splice variant, and the prevalence and pattern of splicing differ markedly across tissues and across vertebrate species, making it a near-universal feature of eukaryotic gene expression rather than an exception.

Evidence & guidelines

The split-gene model rests on the direct demonstration that mRNA sequences are discontinuous with their genes, and the scope of alternative splicing has been quantified by transcriptome-wide studies showing both its prevalence and its evolutionary divergence across species.

History

In 1977 two groups independently showed that adenovirus mRNAs were spliced from segments separated on the genome, revealing that genes can be interrupted by introns; this overturned the colinearity assumption and led to the concept of the split gene. Subsequent work established splicing as a regulated process and alternative splicing as a widespread generator of proteome diversity.

Key figures

• Phillip Sharp
• Richard Roberts
• Susan Berget
• Benjamin Blencowe

Seminal works

• berget-1977
• nilsen-graveley-2010
• barbosa-morais-2012

Frequently asked questions

Are introns just useless filler?

No. Introns are removed from the mature message, but they carry splicing signals, can host regulatory elements and non-coding RNAs, and their presence enables alternative splicing, so they are functionally important parts of gene anatomy.

How can one gene make several proteins?

Through alternative splicing: by including or skipping particular exons, the cell assembles different mature mRNAs from the same gene, each of which can be translated into a distinct protein isoform.