Exon
Exons are gene segments that form mature RNA after splicing.
An exon is a segment of a gene that remains in the final mature RNA molecule after introns are cut out and discarded during RNA splicing. The name applies to both the DNA version within the gene and the matching sequence in the RNA transcript. During splicing, exons are chemically linked together to build the mature RNA, and the complete collection of all exons in a species is called the exome.
The word "exon" is a shortened form of "expressed region," introduced by biochemist Walter Gilbert in 1978. He argued that the old idea of a cistron should be replaced by a transcription unit where some regions (introns) are lost from the mature messenger RNA, while others (exons) are kept and expressed. Originally, this definition applied only to protein-coding transcripts that are spliced before translation. Later, the term was extended to include sequences removed from ribosomal RNA, transfer RNA, and other non-coding RNAs, and it was also used for RNA molecules that come from different parts of the genome and are joined together by trans-splicing.
In terms of genome size and distribution, unicellular eukaryotes like yeast have few or no introns, but metazoans—especially vertebrates—contain large amounts of non-coding DNA. In the human genome, for example, only 1.1% is made up of exons, while 24% is introns and 75% is intergenic DNA. This difference makes whole exome sequencing cheaper and simpler than whole genome sequencing, which is useful in precision medicine. The wide variation in genome size across life forms is known as the C-value enigma. According to data from GenBank in 2002, the average protein-coding gene across all eukaryotes had 5.48 exons, and each exon typically encoded 30 to 36 amino acids. The longest human exon is 11,555 base pairs long, but some exons are only 2 base pairs, and a single-nucleotide exon has been found in the Arabidopsis genome. In humans, most non-coding RNAs also contain multiple exons, just like protein-coding mRNAs.
For protein-coding genes, exons include both the protein-coding sequence and the untranslated regions (UTRs) at the 5′ and 3′ ends. Often the first exon contains the 5′-UTR and the start of the coding sequence, but some genes have exons that consist only of UTR regions—meaning UTRs can contain introns. Some non-coding RNA transcripts also have exons and introns. Mature mRNAs from the same gene do not have to in
- field
- Molecular biology, genetics
- known_for
- Coining the term 'exon' as a shortening of 'expressed region'
- notable_work
- Definition of exons as regions expressed in mature messenger RNA, alternating with introns
Lore & Background
The term exon was coined by American biochemist Walter Gilbert in 1978 as a shortening of 'expressed region'. He proposed that the notion of the cistron be replaced by a transcription unit containing regions lost from mature messenger (introns) alternating with regions that will be expressed (exons). This definition originally applied to protein-coding transcripts spliced before translation, but later came to include sequences removed from rRNA, tRNA, and other non-coding RNA, as well as RNA molecules from different genome parts ligated by trans-splicing.
Reader's Guide
Exons are fundamental to understanding gene structure and expression. In protein-coding genes, exons include both the coding sequence and untranslated regions (UTRs). Mature mRNAs from the same gene need not include the same exons due to alternative splicing, which removes different introns. The exome—the entire set of exons—is much smaller than the genome; in humans, only 1.1% of the genome is spanned by exons, compared to 24% introns and 75% intergenic DNA. This makes whole exome sequencing a smaller and less expensive challenge than whole genome sequencing, providing practical advantages in precision medicine. Exon trapping is a molecular biology technique that exploits intron-exon splicing to find new genes. Splicing can be experimentally modified using Morpholino antisense oligos to block splice-directing snRNPs or to alter splicing patterns. Common misuse of the term includes claiming that exons code for protein or are translated, which omits exons that become part of non-coding RNA or untranslated regions.
Did You Know?
- The term exon is a shortening of 'expressed region' and was coined by Walter Gilbert in 1978.
- In the human genome, only 1.1% of the genome is spanned by exons, while 24% is in introns.
- Across all eukaryotic genes in GenBank (as of 2002), there were on average 5.48 exons per protein-coding gene.
- A single-nucleotide exon has been reported from the Arabidopsis genome.
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