Introns vs Exons- Definition, 12 Major Differences, Examples

Introns and exons are two types of nucleotide sequences that are found within a gene. A gene is a segment of DNA that contains the information for making a protein or a functional RNA molecule. However, not all parts of a gene are used for this purpose. Some parts are coding, meaning that they specify the amino acid sequence of a protein or the structure of an RNA molecule. These parts are called exons. Other parts are non-coding, meaning that they do not contribute to the final product of a gene. These parts are called introns.

Introns and exons are present in most genes of eukaryotic organisms, which include animals, plants, fungi, and protists. They are also found in some viruses that infect eukaryotic cells. However, they are absent in prokaryotic organisms, such as bacteria and archaea, which have simpler and more compact genomes.

The presence of introns and exons in eukaryotic genes has important implications for gene expression, which is the process by which the information in a gene is used to make a protein or an RNA molecule. Before a gene can be expressed, it has to be transcribed into a precursor messenger RNA (pre-mRNA) molecule by an enzyme called RNA polymerase. The pre-mRNA molecule contains both introns and exons, but only the exons carry the useful information for making a protein or an RNA molecule. Therefore, the introns have to be removed from the pre-mRNA molecule by a process called RNA splicing. RNA splicing involves cutting out the introns and joining together the exons to form a mature messenger RNA (mRNA) molecule that can be translated into a protein or used as a functional RNA molecule.

RNA splicing is carried out by a complex of proteins and small RNA molecules called the spliceosome. The spliceosome recognizes specific sequences at the boundaries of introns and exons and catalyzes the cleavage and ligation reactions that remove the introns and join the exons. The introns are then degraded or recycled, while the exons form a continuous coding sequence in the mRNA molecule.

The presence of introns and exons in eukaryotic genes allows for several advantages over prokaryotic genes that lack them. One advantage is that introns and exons enable alternative splicing, which is a process by which different combinations of exons can be spliced together to form different mRNA molecules from the same gene. This increases the diversity of proteins and RNA molecules that can be produced from a single gene, allowing for more complex and flexible regulation of gene expression. Another advantage is that introns and exons facilitate exon shuffling, which is a process by which exons from different genes can be exchanged or rearranged during recombination events. This allows for new combinations of functional domains in proteins and RNA molecules, enhancing their evolution and adaptation.

In summary, introns and exons are two types of nucleotide sequences that are found within a gene. Introns are non-coding sequences that are removed by RNA splicing, while exons are coding sequences that remain in the mRNA molecule. Introns and exons are present in most eukaryotic genes but not in prokaryotic genes. They play important roles in gene expression, alternative splicing, exon shuffling, and evolution.

Introns are non-coding DNA sequences within a gene that are removed by RNA splicing during maturation of the RNA product. The term ‘intron’ represents the intragenic region which is present within a gene. The term ‘introns’ indicates both the DNA sequences within the gene and the corresponding sequence in RNA transcripts.

Introns are found in the genes of many eukaryotic organisms and also some viruses and are located in most genes including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA). These are, however, not found in prokaryotes.

Introns are common in protein-coding nuclear genes of most jawed vertebrates but they might be rare in some eukaryotic organisms. Similarly, the mitochondrial genomes of jawed vertebrates are almost entirely devoid of introns whereas those in other eukaryotes have many introns.

During the generation of proteins from genes containing introns, RNA splicing occurs as a process of RNA processing that occurs after transcription and before translation. RNA splicing is the removal of introns and joining of exons in a specific order to form a mature mRNA transcript.

There are different types of introns based on their sequence analysis and the genetic and biochemical analysis of RNA splicing methods. The four more common types of introns include:

• Spliceosome introns in nuclear protein-coding genes that are removed by spliceosomes
• tRNA introns in nuclear and archaeal tRNA genes that are removed by proteins
• self-splicing group I introns removed by RNA catalysis
• self-splicing group II introns removed by RNA catalysis

Different introns are also lost and gained throughout evolution as observed in different eukaryotes.

Introns are crucial because they enhance the diversity and complexity of protein synthesis by alternative splicing in which introns take part in important roles. Alternative splicing is a controlled molecular mechanism producing multiple variant proteins from a single gene in a eukaryotic cell.

The level of gene expression is also influenced by the presence of introns. It has been reported that spliced transcripts are exported faster from the nucleus to cytoplasm than the unspliced ones.

However, the existence of introns in the genome might be a burden to some cells, because the cells have to consume a great deal of energy to copy and excise them exactly at the correct positions with the help of complicated spliceosomal techniques.

• Introns are non-coding DNA sequences that do not contribute to the final protein product. They are transcribed into RNA but removed by RNA splicing before translation.
• Introns are found in most eukaryotic genes, especially those that code for proteins. They are also present in some viral and archaeal genes, but not in prokaryotic genes.
• Introns vary in size, number, and location within a gene. Some introns are very short, while others can be several kilobases long. Some genes have only one intron, while others have dozens or even hundreds. Some introns are located near the ends of the gene, while others are interspersed throughout the coding region.
• Introns have specific sequences at their boundaries that mark them for splicing. The most common type of intron has a GU nucleotide pair at the 5` end and an AG nucleotide pair at the 3` end. These are called splice donor and splice acceptor sites, respectively. There is also a branch point sequence near the 3` end that contains an A nucleotide. These sequences are recognized by a complex of proteins and RNA molecules called the spliceosome, which catalyzes the removal of introns and the joining of exons.
• Introns have different types based on their sequence and splicing mechanism. The most common type is the spliceosomal intron, which is removed by the spliceosome as described above. Other types include tRNA introns, which are removed by specific enzymes; group I and group II introns, which are self-splicing and can act as ribozymes; and minor introns, which have different boundary sequences and require a different spliceosome.
• Introns have various functions and roles in gene expression and evolution. Some of the possible functions of introns are:
  • Enhancing the diversity of proteins by allowing alternative splicing, which can produce different isoforms of a protein from the same gene by using different combinations of exons.
  • Regulating gene expression by containing sequences that can act as promoters, enhancers, silencers, or binding sites for transcription factors or other regulatory molecules.
  • Facilitating recombination and gene shuffling by providing sites for crossover events or transposable elements that can rearrange or insert new genetic material.
  • Protecting genes from mutations or damage by acting as buffers or spacers between exons or coding regions.
  • Encoding functional RNAs or peptides that can have biological activities independent of the protein product of the gene.

These are some of the main characteristics of introns that make them an important part of eukaryotic genomes. However, introns also pose some challenges and costs for the cells, such as requiring more energy and time for transcription and splicing, increasing the risk of errors or defects in splicing, and occupying more space in the genome. Therefore, introns are subject to evolutionary pressures that balance their benefits and drawbacks.

Exons are protein-coding DNA sequences that contain the necessary codons or information for protein synthesis. The term `exon` represents the expressed region present in the genome. Exons are found in all organisms ranging from jawed vertebrates to viruses, but they form a small part of the total genome. In the human genome, only 1% of the total genome is composed of exons while the rest is occupied by introns and intergenic DNA.

Exons are separated by non-coding introns in genes that code for proteins. During RNA splicing, the introns between the exons are removed to connect two different exons that then code for messenger RNA (mRNA). The resulting mRNA has only exons and untranslated regions (UTRs) where the exons carry the codons that code for various proteins. The UTRs are important for regulating the stability, localization and translation of mRNA.

Exons can also undergo alternative splicing and exon shuffling to increase the diversity of proteins produced from a single gene. Alternative splicing is a process where exons can be arranged in different sequences or configurations to produce different mRNAs and proteins. Exon shuffling is a process where exons or sister chromosomes are exchanged during recombination to create new combinations of exons. These processes allow for the evolution of new functions and interactions of proteins.

Exons can also be created from introns by a process called exonization. Exonization is a process where some introns acquire splice sites and become exons. This can happen due to mutations, insertions or duplications of DNA sequences. Exonization can also increase the complexity and diversity of gene expression.

Exons are crucial for protein synthesis as they are the regions that carry the information for making proteins. They also play a role in regulating gene expression and protein interactions by undergoing various processes such as alternative splicing, exon shuffling and exonization. Exons are therefore essential for the function and evolution of living organisms.

• Exons are protein-coding DNA sequences that require the necessary codons or information necessary for protein synthesis. The term ‘exon’ represents the expressed region present in the genome.
• The genes in eukaryotes are formed of coding exons separated by non-coding introns. During RNA splicing, the introns between the exons are removed to connect two different introns that then code for messenger RNA.
• The entire set of all exons present in the genome of the organisms is termed exosome. In genes coding for proteins, exons include both the protein-coding sequence and the 5’ and 3’ untranslated regions.
• Once these genes are transcribed, the resulting RNA has both exons and introns. The introns are then removed by RNA splicing resulting in mature mRNAs. The mature mRNA transcripts thus have exons and untranslated regions where the exons form a small part of the entire sequence.
• Exons are present in all organisms ranging from jawed vertebrates to viruses. In the human genome, only 1% of the total genome is formed of exons while the rest is occupied by introns and intergenic DNA.
• Sometimes, some introns are converted into exons by the process of exonization. Exonization is a process where a new exon is created from a previously non-coding sequence, such as an intron or a transposable element.
• Exons are crucial in protein synthesis as they are regions carrying codons that code for various proteins. The presence of exons and introns allows the process of alternative splicing that increases the variety of proteins produced from a single gene.
• Alternative splicing allows exons to be arranged in different sequences where different configurations result in different proteins. A process similar to alternative splicing is exon shuffling where exons or sister chromosomes are exchanged during recombination.
• Alternative splicing occurs commonly in a human gene that codes for a transmembrane protein involved in the regulation of potassium entry in the hair cell. This gene consists of 35 exons which can combine in different ways or configuration to form over 500 mRNAs by the reshuffling of about one to eight exons.

Introns and exons are two types of DNA sequences that are present in the genes of eukaryotic organisms. Introns are non-coding regions that are removed by RNA splicing during the maturation of the RNA product, while exons are coding regions that contain the necessary information for protein synthesis. Introns and exons have different characteristics and functions in the genome and in the process of gene expression. Introns and exons also participate in various mechanisms that increase the diversity and complexity of proteins produced from a single gene, such as alternative splicing and exon shuffling. Introns and exons are thus important components of the eukaryotic genome that play vital roles in the regulation and evolution of gene expression.

To illustrate the concept of introns and exons, let`s look at some examples of genes that contain them.

• The human dystrophin gene, which is involved in muscle function, is one of the largest genes in the human genome. It has 79 exons and 78 introns, spanning over 2.4 million base pairs of DNA. The pre-mRNA transcript of this gene is about 2.5 million nucleotides long, but after splicing, the mature mRNA is only about 14,000 nucleotides long.
• The human beta-globin gene, which is part of the hemoglobin protein that carries oxygen in red blood cells, has three exons and two introns. The pre-mRNA transcript of this gene is about 1,600 nucleotides long, but after splicing, the mature mRNA is only about 600 nucleotides long.
• The human immunoglobulin heavy chain gene, which is part of the antibody protein that fights infections, has multiple exons and introns that can be rearranged by a process called V(D)J recombination. This process allows the generation of a large diversity of antibodies from a limited number of genes. The pre-mRNA transcript of this gene can vary in length depending on which exons are included or excluded by alternative splicing.

These examples show that introns and exons are common features of eukaryotic genes, and that they play important roles in gene expression, protein synthesis, and genetic diversity.

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