Molecular genetics · BEGINNER LESSON
Exon
An exon is a segment of a pre-mRNA molecule that remains after splicing, when introns are removed and exons are joined to form mature mRNA. In exon-definition splicing, the spliceosome recognizes an exon as a unit by checking the splice sites on both sides of that exon.
What an exon is
During messenger RNA (mRNA) maturation, pre-mRNA is cut and stitched by the spliceosome, a large RNA-protein machine. Introns are removed, and exons are the parts that are joined together and retained in the mature message. In the sources, an exon is also the unit recognized in a splicing mechanism called exon definition, where the machinery checks the pair of splice sites around an exon rather than focusing first on the intron.
[2] The birth of new exons: Mechanisms and evolutionary consequences - PMC [3] Exon Definition Facilitates Reliable Control of Alternative ...
How exon definition works
In exon definition, splicing machinery first searches for a pair of splicing sites in each exon. The adjacent exons must be properly recognized for the intron between them to be removed. The sources note that this can make exons act like functional units and can improve splicing accuracy, especially for genes with small exons and large introns. A closely related mechanism, intron definition, instead uses the intron as the unit of recognition.
[1] Exon definition as a potential negative force against intron ... [3] Exon Definition Facilitates Reliable Control of Alternative ...
Why exon definition matters in evolution and regulation
Because exon definition depends on exon size, it can constrain how long an exon can be. If an intron is lost, two flanking exons fuse into a larger exon, and that larger exon may be skipped if it exceeds what exon-definition splicing handles well. The sources also describe exonization, where a sequence that was not originally an exon becomes one; this often happens through alternative splicing so the original transcript can still be made while a new exon-containing form is tested.
[1] Exon definition as a potential negative force against intron ... [2] The birth of new exons: Mechanisms and evolutionary consequences - PMC [3] Exon Definition Facilitates Reliable Control of Alternative ...
Limitations and boundaries
The sources emphasize that exon definition is not universal. It is common in vertebrates with small exons and large introns, while intron definition is common in some lower eukaryotes. Exon definition limits exon size but does not directly limit intron size; intron definition does the opposite. New exon-containing variants are often rare, and many predicted exonized sequences are not functional.
[1] Exon definition as a potential negative force against intron ... [2] The birth of new exons: Mechanisms and evolutionary consequences - PMC [3] Exon Definition Facilitates Reliable Control of Alternative ...
SEE THE IDEA
Exon in exon-definition splicing
The starting RNA transcript contains both exons and introns.
The spliceosome checks the splice sites on both sides of an exon.
If the exon is properly defined, the surrounding intron can be removed.
Exons are joined to make the final message used for protein production.
pre-mRNA -> exon recognition -> splicing decision -> mature mRNA
Exon definition treats the exon as the unit being recognized before an intron is removed.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.