Exon

An exon is a segment of a gene that codes for a portion of the final mature RNA (especially mRNA) that is retained after RNA splicing. Exons contain the coding sequences (and sometimes non-coding UTRs) that are ultimately expressed as part of a protein or functional RNA product.

Key Features:

  • Exons are interspersed with introns (non-coding sequences) in eukaryotic genes.
  • During RNA processing, introns are removed and exons are spliced together to form mature RNA.
  • All protein-coding genes have exons, but not all exons necessarily encode protein—some are part of untranslated regions (UTRs) at the ends of mRNA.

Functions of Exons:

FunctionDescription
Protein codingContain sequences that translate into amino acids
mRNA stability and regulation5′ and 3′ untranslated exons help regulate translation and mRNA lifespan
Alternative splicingExons can be selectively included or excluded to create multiple protein variants from one gene

Exons vs. Introns:

FeatureExonIntron
Retained in mRNA?YesNo (removed during splicing)
Protein-coding?Often (but not always)No
Found inEukaryotic and viral genesEukaryotic genes only
RoleEncodes part of the final RNASpacer/intervening sequences

Biological Significance:

  • Mutations in exons can directly affect protein structure and function (e.g., missense, nonsense, or frameshift mutations).
  • Alternative splicing of exons increases protein diversity.
  • Exon skipping can cause or correct genetic disorders (e.g., some treatments for Duchenne muscular dystrophy work by skipping faulty exons).

Summary Table:

PropertyDescription
DefinitionGene segment that remains in mature RNA
Retained in mRNA?Yes
May encode protein?Yes (in coding regions)
Found InEukaryotic and viral genes
RoleProtein coding, regulation, alternative splicing