An exon is any region of DNA within a gene, that is transcribed to the final messenger RNA (mRNA) molecule, rather than being spliced out from the transcribed RNA molecule. Exons of many eukaryotic genes interleave with segments of non-coding DNA (introns). The term "exon" was coined by Walter Gilbert in 1978. The general structure of a section of DNA Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the biological development of a cellular form of life or a virus. ... The life cycle of an mRNA in a eukaryotic cell. ... In genetics, splicing is a modification of genetic information after transcription, in which introns are removed and exons are joined. ... Transcription is the process through which a DNA sequence is enzymatically copied by an RNA polymerase to produce a complementary RNA. Or, in other words, the transfer of genetic information from DNA into RNA. In the case of protein-encoding DNA, transcription is the beginning of the process that ultimately... Diagram of the location of introns and exons within a gene. ... Walter Gilbert Walter Gilbert (born March 21, 1932) is an American physicist, biochemist, entrepreneur, and molecular biology pioneer. ...

Function

In many genes, each exon contains part of the open reading frame (ORF) that codes for a specific portion of the complete protein. 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). This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... An open reading frame or ORF is any sequence of DNA or RNA that can be translated into a protein. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Exons are the regions of DNA within a gene that are not spliced out from the transcribed RNA and are retained in the final messenger RNA (mRNA) molecule. ...

To the right is a diagram of an heterogeneous nuclear RNA (hnRNA), which is an unedited mRNA transcript, or pre-mRNAs. Exons can include both sequences that code for amino acids (red) and untranslated sequences (grey). Stretches of unused sequence called introns (blue) are removed, and the exons are joined together to form the final functional mRNA. 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). Image File history File links A corrected version of Image:32 gene. ... Pre-mRNA (preliminary mRNA) is a single strand of ribonucleic acid (RNA), synthesized from the DNA in the nucleus of a cell by the process transcription. ... In chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups. ... Diagram of the location of introns and exons within a gene. ... The life cycle of an mRNA in a eukaryotic cell. ...

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 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. Some mRNA transcripts have exons with no ORF's and thus are sometimes referred to as non-coding RNA. In eukaryotic genetics, the 5 UTR (read as 5 prime UnTranslated Region) is a particular section of messenger RNA (mRNA). ... In genetics, the 3 UTR (read as 3 prime untranslated region) is a particular section of messenger RNA (mRNA). ... Transcript can have several meanings depending on the context used. ... Various modes of alternative splicing Alternative splicing is the process that occurs in eukaryotes in which the splicing process of a pre-mRNA transcribed from one gene can lead to different mature mRNA molecules and therefore to different proteins. ... A non-coding RNA (ncRNA) is any RNA molecule that functions without being translated into a protein. ...

Exonization is the creation of a new exon, as result of mutations in intronic sequences. Diagram of the location of introns and exons within a gene. ...

Polycistronic messages have multiple ORF's in one transcript and also have small regions of untranslated sequence between each ORF. Messenger RNA (mRNA) is said to be polycistronic when it contains the genetic information to translate more than one protein. ...

Experimental approaches that utilise exons

Exon trapping or 'gene trapping' is a molecular biology technique that exploits the existence of the intron-exon splicing to find new genes. 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. A scientist knows that a new gene has been trapped when the reporter gene is expressed. Exon trapping is a molecular biology technique to identify potential exons in a fragment of eucaryote DNA of unknown intron-exon structure. ... Molecular biology is the study of biology at a molecular level. ... In genetics, splicing is a modification of genetic information prior to translation. ... In molecular biology, a reporter gene (often simply reporter) is a gene that researchers attach to another they wish to study in cell culture, animals or plants. ... 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 factors) to enhance transcription levels of genes (hence the name) in a gene-cluster. ...

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. This has become a standard technique in 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. Morpholino oligos are an antisense technology used to block access of other molecules to specific sequences within nucleic acid molecules. ... Views of a Foetus in the Womb, Leonardo da Vinci, ca. ...

References

• Definition of exon
• Gilbert W (1978) Why genes in pieces? Nature 271:501
• Zhang MQ (1998) Statistical features of human exons and their flanking regions. Hum Mol Genet 7:919932