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Key difference between Exons and Introns is that introns stay inside the nucleus, keeping the DNA safe in the genes. While, Exons leave the nucleus to be translated into a protein. Nucleic acid sequences represented in the RNA molecule are referred to as exons. The nucleotide sequences found in genes that are deleted during the RNA splicing process are known as introns. Exons are also referred to as coding sections, whereas introns are referred to as non-coding portions.
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| Table of Contents |
Key Terms: Genes, Nucleic acid, Transcription, Translation, Nucleotides, Nucleosides, RNA Splicing
What are Introns?
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Introns are nucleotide regions in DNA and RNA that do not code for proteins and are deleted by RNA splicing during the precursor messenger RNA (pre-mRNA) stage of mRNA maturation. Introns are found in a wide variety of genes that create RNA in most living species, including viruses, and can range in size from tens to thousands of base pairs.
Introns can be classified into four different types:
- Spliceosomes delete introns from protein-coding genes.
- Introns, which are eliminated by proteins, are found in tRNA genes.
- self-splicing introns use guanosine-5'-triphosphate (GTP) or another nucleotide cofactor to remove themselves from mRNA, tRNA, and rRNA precursors (Group 1)
- Self-splicing introns are introns that may delete themselves without the use of GTP (Group 2)
What are Exons?
Exons are nucleotide sequences that are conserved in DNA and RNA and are used to generate mature RNA. Transcription is the process of making mRNA from DNA as a template. In a process known as translation, mRNA collaborates with ribosomes and transfer RNA (tRNA), both of which are found in the cytoplasm, to produce proteins. In addition to any protein-coding sequences, exons normally include both the 5' and 3' untranslated regions of mRNA.
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Difference Between Exons and Introns
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The major differences between exons and introns are tabulated below:
Parameters | Introns | Exons |
|---|---|---|
| Definition | Non-coding DNA sequences inside a gene are eliminated during the maturation of the RNA product by RNA splicing. | Exons are DNA sequences that code for proteins and require the required codons or information for protein synthesis. |
| Type of sequence | Non-coding sequences that do not code for any protein are known as introns. | Exons are protein-coding sequences that are responsible for the production of certain proteins. |
| Location in the DNA | In a DNA sequence, introns are found between two exons. | Exons are protein-coding sequences that are found between two introns or between untranslated regions. |
| Distribution | Only present in eukaryotic genomes. | Present in both eukaryotic and prokaryotic genomes. |
| Location in the cell | After being spliced out of the mRNA transcript during RNA processing, introns remain in the nucleus. | After the mature mRNAs are generated, exons exit the nucleus and travel to the cytoplasm. |
| Present in | Introns can be found in DNA and mRNA transcripts, although they are not seen in mature mRNAs. | DNA, mRNA transcripts, and mature RNAs all contain exons. |
| Conserved | Intron sequences are nearly identical to exon sequences in terms of conservation. The process of exonization may turn some introns into exons. | Exon sequences are well conserved. |
| Involved | Introns are not involved in protein synthesis. | Exons are involved in protein synthesis. |
| Quantity | The nuclear genome has more introns than exons. | Exons make up a smaller proportion of the nuclear genome than introns. |
| Human genome | Introns make up 24% of the human genome. | Exons account for 1% of the human genome. |
| Alternative splicing | Alternative splicing is used to eliminate introns. | After alternative splicing, two or more exons are linked. |
| Novel genes formation | Short non-coding sections of introns may grow into real functioning genes through a continual evolutionary process, resulting in novel genes. | Exons can be arranged in a variety of ways, resulting in distinct sequences that code for different proteins. |
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Functions of Introns
While introns were once thought to be "junk DNA" and still are to some extent, it has been shown that introns play a vital function in gene regulation and expression. Because introns lengthen genes, there's a higher chance of crossing over and recombination between sister chromosomes.
Due to duplications, deletions, and exon shuffling, this increases the genetic variation and can result in novel gene variations. Alternative splicing is also possible with introns. Because exons can be constructed in a variety of ways, a single gene can encode several proteins.
Introns contain a variety of splicing-related regions, such as spliceosome recognition sites. These locations aid the spliceosome in distinguishing between introns and exons. Small locations in the nucleolus recognize nucleolar ribonucleoproteins (snRNPs). A spliceosome is made up of several different snRNPs that are involved in mRNA splicing. Three phases are involved in the splicing process.
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Functions of Exons
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Exons are regions of DNA that code for proteins. Exons that code for different domains of a protein are called exons. The gene might be encoded by a single exon or a group of exons spliced together. Exon shuffling, which occurs when exons and introns are present, allows for more molecular evolution. When exons on sister chromosomes are switched during recombination, exon shuffling occurs. New genes can be formed as a result of this.
Exons also allow for alternative splicing, which allows for the translation of several proteins from a single gene. When the introns are eliminated, this technique permits the exons to be organized in multiple ways. The full removal of an exon, the inclusion of part of an exon, or the inclusion of part of an intron are all possible combinations.
Alternative splicing can happen in the same place to produce different forms of a gene with a similar function, like the human slo gene, or it can happen in different cell or tissue types, like the mouse alpha-amylase gene. Alternative splicing and abnormalities in alternative splicing have been linked to a variety of illnesses, including cancer and alcoholism.
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Things to Remember
- Exons can be found in a wide range of species, from jawed vertebrates to viruses.
- As seen in several eukaryotes, distinct introns are lost and added throughout evolution.
- In the presence of introns, gene expression is considerably increased.
- Exon shuffling is a recombination process in which exons or sister chromosomes are switched, similar to alternative splicing.
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Sample Questions
Ques. What happens if an intron is not removed? (2 Marks)
Ans. Because introns proliferate with each cell division, cells must delete introns to produce the final messenger RNA (mRNA) product. Organisms must expend energy to eliminate them. The spliceosome removes introns from the pre-mRNA and splices exons back together during the splicing process. The RNA would be translated into a nonfunctional protein if the introns were not deleted. Before the RNA migrates to the cytoplasm, it undergoes splicing in the nucleus.
Ques. What are the functions of Introns? (3 Marks)
Ans. Introns serve a significant significance from this standpoint. They act as recombination hotspots in the generation of novel exon combinations. In other words, they're in our genes because they've been employed as a speedier way to put new genes together during evolution.
Introns are significant because alternative splicing, in which introns play a part, greatly increases the protein repertoire or variation. In a eukaryotic cell, alternative splicing is a controlled biological mechanism that produces numerous variant proteins from a single gene.
Ques. Why are introns important? (2 Marks)
Ans. Introns play a crucial role in gene expression and regulation. Introns are transcribed by the cell to aid in the formation of pre-mRNA. Introns can also play a role in determining where specific genes are translated. When researchers remove intronic regions artificially, the expression of a single gene or a group of genes can be reduced.
Ques. What happens if introns are mutated? (3 Marks)
Ans. Large portions of intronic DNA may be retained by the mRNA as a result of mutations in these regions, or entire exons may be spliced out of the mRNA. These modifications may lead to the development of a nonfunctional protein.
The splice site is used to separate an intron from an exon. Introns contain functional polymorphisms that can affect the expression of the genes that host them, in addition to pathogenic mutations in the strict sense. Some of these intronic variations may potentially confer disease vulnerability or alter the genotype-phenotype link in other ways.
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Ques. How do introns work? (2 Marks)
Ans. Any nucleotide sequence within a gene that is deleted by RNA splicing during the maturation of the final RNA product is referred to as an intron (for the intragenic region). In other words, introns are noncoding portions of an RNA transcript of the DNA that encodes it that are removed before translation by splicing.
Ques. What if exons are removed? (2 Marks)
Ans. Exon skipping does not eliminate the mutation; it only allows biological systems to disregard it while creating new proteins. As a result, the reading frame that was disrupted is restored. Even though it contains internal deletions, the protein called for is largely functional. Exons code for specific proteins, whereas introns are non-coding (sometimes known as "junk DNA") and are eliminated during RNA splicing. To make messenger RNA, the remaining exons are covalently bonded together.
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Ques. Do exons contain stop codons? (2 Marks)
Ans. In addition to any protein-coding sequences, exons normally include both the 5' and 3' untranslated regions of mRNA, which contain start and stop codons. Yes, start and stop codons can be found within exons, but if we look closely, we can observe that the stop codon is not present within the same frame of the transcript (joining off all coding parts of a nucleotide).
Ques. What is exon inclusion? (2 Marks)
Ans. It allows a gene to be translated into numerous isoforms (or mRNA transcripts), which improves an organism's phenotypic complexity without adding to its genetic complexity. Exon-inclusion ratio (EIR), commonly known as percent spliced in (PSI), is a prominent metric for determining alternative splicing events.
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