Introns vs Exons: What Sets Them Apart in Biology?
Any nucleotides sequence within a gene that is removed by RNA splicing of the final product during maturation is known as an intron. The final mature RNA produced by that gene is encoded by exons.
So let's discuss intron and exon in detail.
A gene containing coding regions unknown exons are interrupted by non-coding regions known as introns. The DNA region between exons and introns is encoded by exons proteins. In the coding region, only the eukaryotes contain introns.
In eukaryotes, both exons and introns are transcribed into the mRNA primary transcript.
Exons are termed nucleic acid sequences represented in the RNA molecule. Introns can be defined as the nucleotide sequences that are found in the genes that are removed by the process of RNA splicing. We can also say that exons are coding areas, whereas introns are non-coding areas. Both Roberts and Phillip Sharp discovered introns and exons respectively. The introns change their sequences frequently with time, whereas the exon sequences are highly conserved.
What are Introns?
Introns play the role of intervening sequences between two exons found in eukaryotes. They do not directly code for proteins. They are removed before the mRNA forms proteins. Therefore, these introns undergo the process of splicing.
Introns are the non-coding parts of the nucleotides and aren't highly conserved. Therefore, it's essential to get rid of introns to stop the formation of incorrect proteins.
The word intron means ‘In the Nucleus’. Thus, the Universal feature in introns is to remove the splicing within the nucleus by RNA. The sequence of nucleotides found in RNA and DNA that interrupts the sequence of the gene is known as an intron. Introns are found in the mRNA, primary transcript, and intergenic regions of the gene. Hence, mature RNA lacks introns on the other hand RNA splicing mechanism lacks in prokaryotes.
What are Exons?
Exons are the coding sequences that code for the amino acid sequence of the protein. The exons are transcribed into mature mRNA after post-transcriptional modification. These are highly conserved sequences, i.e., they are not changing frequently with time.
Splicing is the process of removing introns. The production of different combinations of amino acids is promoted by alternative splicing by combining different combinations of exons together. Therefore the Amino acid sequence of the polypeptide exons is responsible.
Function of Introns
While introns were initially – and to an extent still are – considered ‘junk DNA’, it's been shown that introns likely play a crucial role in regulation and organic phenomenon.
As introns cause a rise in gene length, this increases the likelihood of crossover and recombination between sister chromosomes. This increases genetic variation and may end in new gene variants through duplications, deletions, and exon shuffling. Introns also allow for alternative splicing. This allows one gene to encode multiple proteins because the exons are often assembled in multiple ways.
The RNA polymerase makes a copy of the whole gene during transcription, both introns, and exons, into the initial mRNA transcript referred to as pre-mRNA or heterogeneous nuclear RNA (hrRNA). As introns aren't transcribed, they need to then be removed before translation can occur. The excision of introns and therefore the connection of exons into a mature mRNA molecule occurs within the nucleus and is understood as splicing.
Introns contain a variety of sequences that are involved in splicing including spliceosome recognition sites. These sites help the spliceosome to identify the boundary between the introns and exons. Nucleolar ribonucleoproteins (snRNPs) are recognised by small sites themselves. There are a variety of snRNPs involved in mRNA splicing which combine to create a spliceosome. The splicing takes place in three steps.
Structure and Function of Exons
In protein-coding genes, the exons include both the protein-coding sequence and therefore the 5′- and 3′-untranslated regions (UTR). Often the primary exon includes both the 5′-UTR and therefore the first a part of the coding sequence, but exons containing only regions of 5′-UTR or (more rarely) 3′-UTR occur in some genes, i.e. the UTRs may contain introns. Some non-coding RNA transcripts even have exons and introns.
Mature mRNAs originating from an equivalent gene needn't include equivalent exons, since different introns within the pre-mRNA are often removed by the method of another splicing. Exonization is the creation of a replacement exon, as a result of mutations in introns.
Some of the important differences between introns and exons are the following:
Difference between Introns and Exons
Conclusion
This is all about the meaning, explanation, and differences between introns and exons. Concentrate on their structural features and functions to grab hold of this biology topic and establish your conceptual foundation.
FAQs on Introns vs Exons: What Sets Them Apart in Biology?
1. What is the fundamental difference between an intron and an exon?
The fundamental difference is that exons are the coding sequences within a gene that are expressed to form proteins, while introns are the non-coding, intervening sequences. During gene expression, introns are removed, and exons are joined together to create the final messenger RNA (mRNA) that is translated into a protein.
2. What happens to introns and exons during the process of transcription in eukaryotes?
During transcription, the entire gene, including both introns and exons, is copied into a preliminary transcript called pre-mRNA or heterogeneous nuclear RNA (hnRNA). Following this, a crucial modification process called splicing occurs, where introns are precisely cut out, and the exons are ligated (joined) together to form a continuous, shorter, mature mRNA molecule ready for translation.
3. Why is it essential for introns to be removed before protein synthesis?
It is essential to remove introns because they do not contain the correct code for building a protein. If introns were left in the mRNA, the ribosome would read them during translation, leading to an incorrect sequence of amino acids. This would result in a non-functional or completely wrong protein, which could be harmful to the cell. Their removal ensures the integrity and correct structure of the final polypeptide chain.
4. What is the concept of a 'split gene' in biology?
The concept of a 'split gene' refers to the genetic arrangement, primarily in eukaryotes, where the coding sequences (exons) are interrupted by non-coding sequences (introns). This contrasts with the continuous genes found in most prokaryotes. The discovery of split genes was a landmark in molecular biology, revealing that genetic information is not always colinear with the protein it codes for and highlighting the importance of post-transcriptional modification.
5. If introns are non-coding, do they serve any biological function?
Yes, while once considered 'junk DNA', introns are now known to have several important functions. These include:
- Gene Regulation: Some introns contain sequences that can enhance or suppress the expression of a gene.
- Alternative Splicing: The presence of introns allows for a process called alternative splicing, where different combinations of exons from the same gene can be joined together. This enables a single gene to produce multiple distinct proteins.
- Evolution: Introns facilitate a process called exon shuffling, which allows for the creation of new genes and proteins over evolutionary time, driving genetic diversity.
6. How does alternative splicing use the exon-intron structure to increase protein diversity?
Alternative splicing is a regulated process that occurs after transcription where specific exons of a gene may be included or excluded from the final processed mRNA. By treating certain exons as 'optional', a cell can generate different mRNA transcripts from a single gene. Each of these transcripts is then translated into a different version of a protein, known as an isoform, with potentially different functions. This dramatically expands the proteome without needing a larger number of genes.
7. Are introns and exons found in all living organisms?
No, the intron-exon structure is a hallmark of eukaryotic genes (in organisms like plants, animals, and fungi). While they exist in some archaeal and bacteriophage genes, they are largely absent from prokaryotic organisms like bacteria. Prokaryotic genes are typically continuous, meaning their genetic code is not interrupted by non-coding sequences, which simplifies their transcription and translation process.
8. What is the evolutionary advantage of having split genes with introns and exons?
The primary evolutionary advantage is increased potential for creating new, functional proteins through exon shuffling. Introns act as flexible spacers between the functional domains coded by exons. During genetic recombination, the chances of a crossover event happening within a long intron are higher than within a short exon. This allows exons (representing protein domains) to be swapped between different genes, leading to novel combinations and accelerating the evolution of new proteins with new functions.
