An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule after a) portions of a precursor RNA, introns, have been removed by cis-splicing or b) two or more precursor RNA molecules have been ligated by trans-splicing. A nucleic acid is a Macromolecule composed of chains of monomeric Nucleotides In Biochemistry these Molecules carry Genetic information Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into Trans -splicing is a special form of RNA processing in Eukaryotes where Exons from two different primary RNA transcripts are joined end to end and The mature RNA molecule can be a messenger RNA or a functional form of a non-coding RNA such as rRNA or tRNA. Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product A non-coding RNA ( ncRNA) is any RNA molecule that is not translated into a Protein. Ribosomal RNA ( rRNA) is the central component of the Ribosome, the protein manufacturing machinery of all living cells. Transfer RNA (abbreviated tRNA) is a small RNA (usually about 74-95 nucleotides that transfers a specific Amino acid to a growing polypeptide chain at Depending on the context, exon can refer to the sequence in the DNA or its RNA transcript.
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History
The term exon was coined by American biochemist Walter Gilbert in 1978:
The notion of the cistron. Biochemistry is the study of the chemical processes in living Organisms It deals with the Structure and function of cellular components such as Walter Gilbert (born March 21, 1932) is an American physicist, biochemist, and Molecular biology pioneer . must be replaced by that of a transcription unit containing regions which will be lost from the mature messenger—which I suggest we call introns (for intragenic regions)—alternating with regions which will be expressed— exons.
– Walter Gilbert, Nature 9 Feb. Nature is a prominent Scientific journal, first published on 4 November 1869 501/1
This definition was originally made for protein-coding transcipts that are spliced before being translated. The term later came to include sequences removed from rRNA[1] and tRNA[2], and it also was used later for RNA molecules originating from different parts of the genome that are then ligated by trans-splicing[3]. Ribosomal RNA ( rRNA) is the central component of the Ribosome, the protein manufacturing machinery of all living cells. Transfer RNA (abbreviated tRNA) is a small RNA (usually about 74-95 nucleotides that transfers a specific Amino acid to a growing polypeptide chain at
Function
In many genes, each exon contains part of the open reading frame (ORF) that codes for a specific portion of the complete protein. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance An open reading frame ( ORF) is a portion of an organism's Genome which contains a sequence of bases that could potentially encode a Protein Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl However, the term exon is often misused to refer only to coding sequences for the final protein. This is incorrect, since many noncoding exons are known in human genes (Zhang 1998). An exon is a Nucleic acid sequence that is represented in the mature form of an RNA molecule after a portions of a precursor RNA Introns have been
To the right is a diagram of an heterogeneous nuclear RNA (hnRNA), which is an unedited mRNA transcript, or pre-mRNAs. Precursor mRNA ( pre-mRNA) also termed heterogeneous nuclear RNA ( hnRNA) is an immature single strand of messenger ribonucleic acid ( MRNA Exons can include both sequences that code for amino acids (red) and untranslated sequences (grey). In Chemistry, an amino acid is a Molecule containing both Amine and Carboxyl Functional groups In Biochemistry, this Stretches of unused sequence called introns (blue) are removed, and the exons are joined together to form the final functional mRNA. Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product The notation 5' and 3' refer to the direction of the DNA template in the chromosome and is used to distinguish between the two untranslated regions (grey).
Some of the exons will be wholly or part of the 5' untranslated region (5' UTR) or the 3' untranslated region (3' UTR) of each transcript. The five prime untranslated region ( 5' UTR) also known as the leader sequence, is a particular section of Messenger RNA (mRNA and the DNA that codes for The three prime untranslated region (3' UTR is a particular section of Messenger RNA (mRNA The untranslated regions are important for efficient translation of the transcript and for controlling the rate of translation and half life of the transcript. Furthermore, transcripts made from the same gene may not have the same exon structure since parts of the mRNA could be removed by the process of alternative splicing. Alternative splicing is the RNA splicing variation mechanism in which the Exons of the primary gene transcript the Pre-mRNA, are separated and reconnected Some mRNA transcripts have exons with no ORF's and thus are sometimes referred to as non-coding RNA. A non-coding RNA ( ncRNA) is any RNA molecule that is not translated into a Protein.
Exonization is the creation of a new exon, as result of mutations in intronic sequences [1]. Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into
Polycistronic messages have multiple ORF's in one transcript and also have small regions of untranslated sequence between each ORF. Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product
Experimental approaches that utilize exons
Exon trapping or 'gene trapping' is a molecular biology technique that exploits the existence of the intron-exon splicing to find new genes. Exon trapping is a Molecular biology technique to identify potential Exons in a fragment of Eukaryote DNA of unknown Intron - Exon Gene trapping is a high-throughput approach that is used to introduce insertional Mutations across the Mammalian genome Molecular biology is the study of Biology at a molecular level The first exon of a 'trapped' gene splices into the exon that is contained in the insertional DNA. This new exon contains the ORF for a reporter gene that can now be expressed using the enhancers that control the target gene. In Molecular biology, a reporter gene (often simply reporter) is a Gene that researchers attach to another gene of interest in Cell culture In Genetics, an enhancer is a short region of DNA that can be bound with Proteins (namely the Trans-acting factors much like a set of Transcription A scientist knows that a new gene has been trapped when the reporter gene is expressed.
Splicing can be experimentally modified so that targeted exons are excluded from mature mRNA transcripts by blocking the access of splice-directing small nuclear ribonucleoprotein particles (snRNPs) to pre-mRNA using Morpholino antisense oligos[4]. In Molecular biology, a Morpholino is a Molecule used to modify Gene expression. This has become a standard technique in developmental biology. Developmental Biology is the official journal of the Society for Developmental Biology. Morpholino oligos can also be targeted to prevent molecules that regulate splicing (e. g. splice enhancers, splice suppressors) from binding to pre-mRNA, altering patterns of splicing.
See also
References
- ^ Kister, K-P, Eckert,(1987) W. Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product See also Five prime untranslated region, Three prime untranslated region UTR, which stands for Untranslated Region refers to either of two sections on Many Genomes have been sequenced and their gene sequences are stored in general DNA sequence databases (e A. , Characterization of an authentic intermediate in the self-splicing process of ribosomal precursor RNa in macronuclei of Tetrahymena thermophila Nucleic Acid research 15:1905-1920
- ^ Valenzuela P, Venegas A, Weinberg F, Bishop R, Rutter WJ. (1978) Structure of yeast phenylalanine-tRNA genes: an intervening DNA segment within the region coding for the tRNA. Proc Natl Acad Sci U S A. 1978 Jan;75(1):190-4.
- ^ The transposition unit of variant surface glycoprotein gene 118 of Trypanosoma brucei. Presence of repeated elements at its border and absence of promoter-associated sequences. Liu AY, Van der Ploeg LH, Rijsewijk FA, Borst P. J Mol Biol. 1983 167(1):57-75 PMID: 6306255
- ^ Morcos, PA (2007). "Achieving targeted and quantifiable alteration of mRNA splicing with Morpholino oligos." (Pubmed). PubMed is a free search engine for accessing the MEDLINE database of citations and abstracts of biomedical research articles Biochem Biophys Res Commun.
- Gilbert, W (Feb 9, 1978). Walter Gilbert (born March 21, 1932) is an American physicist, biochemist, and Molecular biology pioneer "Why genes in pieces?". Nature 271 (5645): 501. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 622185.
- Zhang, MQ (May 1998). "Statistical features of human exons and their flanking regions". Hum Mol Genet 7 (5): 919-32. PMID 9536098.
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