Exons, Coding Sequence, and Genes
Exon is a part or a portion of a gene which codes for a particular amino- acids. In the plant and animal cells, most of the gene sequences are broken by one or more sequences of DNA, and they are named as Introns.
These parts of the gene sequences which are expressed as proteins are called as Exons.
The name is because, exons express, and introns do not express in terms of proteins, as introns come in between or to the interfere with the exons.
Exons are a part of the RNA, which codes for the proteins. when RNA gets transcribed initially it seems to be very large piece of molecule.
It is also very important to known that the RNAs here are the exons. These are the large chunks of the RNA which gets out excised. It is also important to know that exons are said to be excised because they move away.
The exons are the ones which stay in the mature RNA, and codes for the amino acids that are necessary.
What are Exons?
Exon is the particular part of a gene which codes a specific part of a mature RNA, which have been produced by the gene after removing the introns by splicing the RNA.
The term Exon is referred to both DNA sequencing that is present within the genome and the respective sequences that is present in the transcripts of RNA.
During RNA splicing introns are removed and the exons are let to join covalently along with-it ad a part of mature messenger RNA.
As the entire set of genes constitute a genome, the whole set of exons form an exome.
The term exon has been derived due to its capability of expressing the region of the gene.
It was termed by an American biochemist named Walter Gilbert in the year 1978. Where the notion of cistron was replaced by the unit of transcription containing regions that will be lost from the mature messenger RNA, and are called as introns. And the one which is expressed is known as Extron’s.
Contributions of Exons
Even the unicellular eukaryotes such as the yeast, doesn’t have introns or any other metazoans, vertebrate genomes have a large ratio of the non-coding DNA.
For instance if a human genome contains 1.1 percentage of the exons, there will be about 24 percentage of introns along with the remaining 75 percentage as the genome which will be as intergenic DNA.
All the eukaryotic genes in a gene bank have an average of 5.48 exons per gene. This exon encoded more than 30 – 36 amino acids.
The longest exon that is present in the human genome is 11,555 base pairs long, several exons are found just in 2 base pair long genome.
A single nucleotide of exon has been reported from the Arabidopsis genome.
Structure and Function of Exons
Exon is a precursor of messenger RNA. Exon includes both sequences which codes for amino acids and untranslated sequences.
In genes which code for proteins has the exons which includes both the protein coding sequence and the translational regions from 5’ to 3’ end.
Always the first exon includes both of the sequences, i.e., 5’UTR and the first part of the coding sequence, but exons which are containing only the regions of 5’UTR occurs in some genes, which is said that UTR contains introns.
Some of the non-coding transcripts of RNA has both exons and introns. Mature mRNA which originates from the same gene need not include the same exons, because introns present in the pre-mRNA can be removed by using the process of alternative splicing.
Exonization is the creation of the new exon, which is results as the mutation in the introns.
Exon trapping or gene trapping is a technique of molecular biology which exploits an existence of the intron-exon splicing in order to find the new genes.
Initially the trapped gene exon is spliced into an exon which is contained in the insertional DNA.
The new exon forms a ORF for a reporter gene which can express using the enhancers which is able to control the target gene.
Researches found that a new gene have been trapped at the time of expressing the reporter gene.
Splicing may be modified experimentally so that the targeted exons were excluded from the mature mRNA which transcripts by blocking the activity of the splice-directing small nuclear ribonucleoprotein particles to the pre-mRNA using the Morpholino antisense oligos, it has become the stranded technique in the field of developmental biology.
Where as the Morpholino oligos be targeted he prevent the molecules which regulates the splicing of enhancers, and suppressors from binding to pre-mRNA by altering the patterns.
Exon shuffling is a one of the molecular mechanisms which is used for the formation of new genes.
It is one of the process through which two or more exons from the different genes are brought together ectopically, or the exon may be duplicated to create a new exon intron structure.
This technique for first introduced by Walter Gilbert in the year 1978. Where as the existence of introns could play an important role in evolution of proteins.
It has also been found that recombination within the intron can help in assorting the exons independently.
The respective segmental which are present in the middle of the introns can create hotspots for the recombination to shuffle the sequence of exons, which is known as exonic sequences.