Exon (Microbiology For Dummies)

By Microbiology Doctor-dr

Exon- Definition, Structure, Splicing, Process Of Splicing

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Exon is a section of a eukaryotic gene that encodes a portion of the final product of the gene (protein).

Exon = Expressed regions.

Exon is also described as the RNA segment that persists after post-transcriptional modification and is translated into protein or integrated into RNA structure- this section of the gene codes for proteins. 

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• Exons are found solely in eukaryotic genes.
• Exons are a gene's coding domains.
• They encode for proteins and are critical parts of the gene.
• In a gene, exons are separated by introns.
• Splicing is a post-transcriptional process that removes introns (non-coding regions) and joins exons.

Exon Structure

• Exons are nucleotide sequences in DNA or RNA that are expressed.
• Exons are found in both DNA and mature RNA.
• Exon structure comprises both 5' and 3' untranslated regions. This comprises start and stop codons as well as other codons that code for proteins.

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Splicing

Splicing is the process by which non-coding introns are removed from primary mRNA and exons are linked together in the main transcript.

• A pre-mRNA is converted into a mature mRNA throughout this process.
• Splicing happens before translation, before protein synthesis.
• Splicing is necessary because accurate protein cannot be coded without it.
• It is also vital in the control of gene expression and protein expression.
• There are several techniques for splicing, however the process is mostly determined by three major factors:

1. Organism type
2. RNA structure or intron
3. A catalyst is present.

Splicing procedure

1. Pre-mRNA exon-intron junctions have conserved sequences. The 5' end of the pre-mRNA junction has a GU sequence, whereas the 3' end contains an AG sequence.
2. These precise locations are referred to as 5' and 3' splice sites, respectively.
3. Along with these two sites, there is an invariant site 15-45 nucleotides upstream of the 3' slice site. This location is known as Branch point or Branch site because it is rich in A bases.
4. At the corresponding splice sites, the spliceosome identifies these conserved sequences in the pre-mRNA.
5. A spliceosome is a collection of RNA-protein complexes, each of which is made up of tiny nuclear RNA and a protein. This tiny nuclear RNA and protein are referred to as small nuclear ribonucleotideproteins (snRNP).
6. The spliceosome is made up of the following essential snRNPs: U1, U2, U5, and U4-U6. (U4 and U6 are contiguous.)

Pathway Splicing

1. U1 and U2 are involved in the first splicing step. U1 attaches to the 5' splice site, while U2 binds to the invariant or branch point.
2. The remaining snRNPs attach to the already bound snRNPs in the second stage. U5 and U4-U6 connect to the intron region, and so the spliceosome is formed.
3. The spliceosome now loops out the intron, bringing the two ends of the introns close together.
4. U1 and U4 are also released, whereas U6 is attached to both the 5' splice site and U2.
5. The 5' end of the intron is then cleaved and attached to the intron's branch point, which is rich in A.
6. Finally, the 3' end of the introns is cleaved, releasing the intron and allowing it to be degraded further by enzymes. The intron structure is known as a Lariat (loop-like).
7. Finally, the two exons are linked together.
8. The snRNPs are employed for splicing other introns, and the splicing process is repeated for the remaining introns in the pre-mRNA.

Splicing Alternatives

• Alternative splicing is the process of creating distinct mRNA versions by connecting different exons.
• Protein isoforms are produced as a result of alternative splicing.
• This alters the chemical and biological action of proteins.
• This means that a single gene can code for many types of mRNA and proteins.

The following are the most typical forms of alternative splicing:

1. Successive splicing involves the splicing of consecutive introns and the joining of consecutive exons.
2. Exon skipping is a process in which certain exons and their surrounding exons are removed from the pre-mRNA prior to translation.
3. Alternative 5' or 3' splice site - this can be accomplished by connecting exons at the alternative 3' or'splice site.
4. Intron retention occurs when certain introns are maintained in mature mRNA.

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Self-Splicing Introns

Self-splicing is the mechanism by which introns (or RNA) remove themselves from pre-mRNA without the use of precursors or proteins.

It happens through the phosphoester transfer pathway.

• The nucleophile that attacks and connects to the 5' phosphate of the first nucleotide of the intron is the 3'OH of the guanine nucleotide. 
• This allows the 3'OH of the upstream exon's last nucleotide free to attack and connect the 5' phosphate of the downstream exon's first nucleotide. 
• These two phosphoester transfers result in the merging of two exons and the removal of an intron (with the beginning G nucleotide connected to the 5' end). 
• The excised intron is subsequently circularised by the assault of the 3'OH of the intron's final nucleotide on the phosphate between the introns' 15th and 16th nucleotides.
• Further degradation effectively eliminates the intron from the reaction and aids in the prevention of the reverse reaction. 
• It should be noted that phosphoester transfers are easily reversible unless the products (excised intron) are eliminated. 
• During this splicing, there is no increase or reduction in the amount of phosphoester linkages.

Conclusion

• Exons are coding regions in which specific amino acids are encoded.
• Exons play a critical role in protein synthesis.
• Introns are non-coding regions that separate exons.
• The method of splicing is used to remove introns.
• Splicing is accomplished by the usage of cellular machinery known as the Spliceosome.
• Exons are linked together by splicing to generate a complete gene with all coding domains.
• A pre-mRNA gets transformed into a mature mRNA at the end of the splicing process.
• Protein isoforms are formed as a result of alternative splicing, and their biological and chemical activity varies. As a result, one gene produces many types of mRNA.
• Some RNA molecules are capable of self-splicing.
• Splicing is a crucial step in eukaryotes before translation.

Molecular biology for dummies