Exon
An exon is any nucleotide sequence within a gene that is represented in the mature, spliced mRNA and is ultimately translated into protein (or retained in functional non-coding RNAs). After transcription, exons are joined together following the removal of introns to form the continuous open reading frame. The modular nature of exons provides an evolutionary advantage: exon shuffling — the rearrangement of exon modules between genes — is a major mechanism for generating new protein functions and domain architectures.
Human Genome Exon Statistics
| Parameter | Typical Value | Notes |
|---|---|---|
| Average exon length | ~170 bp | Much shorter than average intron (~3,500 bp) |
| Average exons per gene | ~8–9 | Range from 1 to over 300 (titin gene) |
| Total exonic fraction of genome | ~1.5% | Only ~3% includes UTRs |
| Coding exon content | ~1.0% of genome | Translates to protein sequences |
| First exon (5' UTR + start) | Often untranslated partially | Contains Kozak/5'UTR regulatory elements |
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Related Terms
Intron
An intron (intervening sequence) is a non-coding segment of a pre-mRNA transcript that is removed by RNA splicing before the mature mRNA is translated into protein. Introns are excised as lariat-shaped structures by the spliceosome, a large ribonucleoprotein complex that recognises conserved splice-site sequences at intron boundaries. Although historically considered "junk DNA," introns are now understood to play important roles in gene regulation, alternative splicing, and as sources of small non-coding RNAs such as snoRNAs and microRNAs.
Alternative Splicing
Alternative splicing is a regulated process during gene expression in which particular exons of a pre-mRNA transcript are included or excluded to generate multiple distinct mature mRNA isoforms from a single gene. This mechanism dramatically expands proteomic diversity; estimates suggest that over 95% of human multi-exon genes undergo alternative splicing, allowing a genome of ~20,000 genes to produce hundreds of thousands of protein variants. Mis-regulation of alternative splicing is implicated in many diseases including spinal muscular atrophy, various cancers, and neurological disorders.
Central Dogma of Biology
The Central Dogma of Molecular Biology, formulated by Francis Crick in 1958, describes the general flow of genetic information within a biological system: DNA is transcribed into RNA, and RNA is translated into protein. Crick's original statement also specified that information transfer from protein back to nucleic acid does not normally occur, establishing a directional framework for gene expression. The discovery of reverse transcriptase in retroviruses revealed that RNA can also be reverse-transcribed into DNA, representing a known exception to the standard flow while still consistent with Crick's framework.
Coined by Walter Gilbert in 1978 from "ex(pressed)" + "-on". Gilbert used the term to describe the portions of a gene that are expressed (i.e., appear) in the final mRNA, in contrast to the intervening introns that are removed.