Introns And Exons: Definition, Differences, Example

• The term ‘intron’ refers to the intragenic region which is present within a gene.
• ‘introns’ as a term indicates both the DNA sequences within the gene and the corresponding sequence in RNA transcripts.
• Introns can be 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 and transfer RNA. However, they are not found in prokaryotes.
• Introns are quite common in protein – coding nuclear genes of most jawed invertebrates but they might be rare in some eukaryotic organisms.
• Likewise, the mitochondrial genomes of jawed vertebrates are almost entirely devoid of introns whereas those in other eukaryotes have many introns.
• During the process of protein generation from genes containing introns, RNA splicing occurs as a process of RNA processing that occurs after transcription and before translation.
• There are various types of introns based on their sequence analysis and the genetic and biochemical analysis of RNA splicing methods.
• Some rather commonly observed types of introns include;

1. Spliceosome introns in nuclear protein- coding genes that are eliminated by spliceosomes
2. tRNA introns in nuclear and archaeal tRNA genes that are eliminated by proteins
3. Self- splicing group I introns eliminated by RNA catalysis
4. Self-splicing group II introns eliminated by RNA catalysis

• Different introns are also eliminated and gained throughout evolution as observed in different eukaryotes.
• Introns are vital because the types of protein are greatly enhanced by alternative splicing in which introns take part in crucial roles. Alternative splicing is a regulated molecular mechanism producing multiple variants proteins from a singular gene in a eukaryotic cell.
• The degree of gene expression is greatly enhanced in the presence of introns.
• It has also been known that spliced transcripts are exported quicker from the nucleus to cytoplasm than the unspliced ones.
• However, the presence of introns in the genome might be a burden to some cells, because the cells have to consume a huge amount of energy to copy and excise them exactly at the correct positions with the help of complicated spliceosomal techniques.
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Exon Definition

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.

• The term ‘exon’ refers to the expressed region present in the genome.
• The genes in eukaryotes consist of coding exons separated by coding introns.
• During RNA splicing, the introns between the exons are eliminated to connect two different introns that then code for messenger RNA.
• The entire set of all exons that exist in the genome of the organisms are called exosome.
• In genes coding for proteins, exons consist of both the protein- coding sequence as well as the 5’ and 3’ untranslated regions.
• Once these genes are transcribed, the resulting RNA has both exons and introns. The introns are then eliminated by RNA splicing which results in mature mRNAs.
• The mature mRNA transcripts, as a result, have exons and untranslated regions where the exons form a small fraction 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 consists of introns and intergenic DNA.
• Seldom, a few introns are converted into exons by the process of exonization.
• Exons are vital in protein synthesis as they are regions carrying codons that code for multiple kinds of proteins.
• The existence of exons and introns allows the process of alternative splicing that increases the number of proteins produced from a singular gene.
• Alternative splicing leads exons to be arranged in different sequences where the resulting protein is dependent on the configuration.
• A process like alternative splicing is exon switching where exons or sister chromosomes are exchanged during recombination.
• Alternative splicing occurs often in a human gene that codes for a transmembrane protein related to the regulation of potassium entry in the hair cell.
• This gene consists of 35 exons which can combine in various configurations to form over 500 mRNAs by the reshuffling of about one to eight exons.