An exon is a coding region of a gene that
contains the information required to encode a
protein. In eukaryotes, genes are made up of
coding exons interspersed with non-coding introns.
These introns are then removed to make a
functioning messenger RNA (mRNA) that can be
translated into a protein.
3. Exon Structure
Exons are made up of stretches of DNA that will ultimately
be translated into amino acids and proteins. In the DNA of
eukaryotic organisms, exons can be together in a continuous
gene or separated by introns in a discontinuous gene.
When the gene is transcribed into pre-mRNA the transcript
contains both introns and exons. The pre-mRNA is then
processed and the introns are spliced out of the molecule. Mature
mRNAs can be a few hundred to several thousand nucleotides
4. The mature mRNA consists of exons and short untranslated
regions (UTRs) on either end. The exons make up the final
reading frame which consists of nucleotides arranged in triplets.
The reading frame begins with a start codon (usually AUG) and
ends in a termination codon. The nucleotides are arranged in
triplets as each amino acid is coded for by a three-nucleotide
6. Exon Function
Exons are pieces of coding DNA that encode proteins.
Different exons code for different domains of a protein. The
domains may be encoded by a single exon or multiple exons
spliced together. The presence of exons and introns allows for
greater molecular evolution through the process of exon
shuffling. Exon shuffling occurs when exons on sister
chromosomes are exchanged during recombination. This allows
for the formation of new genes.
7. Exons also allow for multiple proteins to be translated from the same
gene through alternative splicing. This process allows the exons to be
arranged in different combinations when the introns are removed. The
different configurations can include the complete removal of an exon, the
inclusion of part of an exon, or the inclusion of part of an intron.
Alternative splicing can occur in the same location to produce different
variants of a gene with a similar role, such as the human slo gene, or it can
occur in different cell or tissue types, such as the mouse alpha-amylase gene.
Alternative splicing, and defects in alternative splicing, can result in a
number of diseases including alcoholism and cancer.