Exon

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Schematic structure of a gene. When a gene is transcribed (DNA → RNA), the introns are spliced out . The m [essenger] RNA is composed of the transcribed sequences of the exon. The exons can be coding, partially coding or non-coding.

As exon (of English. Ex pressed regi on ) the portion of a eukaryotic gene referred to that after the splicing ( splicing retained). In contrast are the introns ( English intr agenic regi ons ), the cut out in splicing and are degraded. The typical human gene contains an average of eight exons with an average internal exon length of 145 nucleotides . Introns are on average more than 10 times as long, and in some cases even significantly longer.

Exons protein-encoding genes contain the open reading frame ( English {open reading frame , ORF) and, in addition to the 5 'and 3' untranslated region ( English region untranslated , UTR) from the terminal exon. Only about 1.5 percent of the total genomic deoxyribonucleic acid (DNA) code for proteins (the haploid human genome amounts to around 23,000 protein-coding genes), while the remainder of the genes for non-coding deoxyribonucleic acids ( English non-coding DNA ) and introns , regulatory DNA and non-coding deoxyribonucleic acids (so-called "junk" DNA). Since many of the protein-coding genes produce more than one protein , including through alternative splicing of the primary transcript ( precursor mRNA , pre-mRNA) of a gene, there are far more than 23,000 different proteins in the human body. It is only during the splicing process that it is decided which DNA sequences are introns and which exons. In other words: Since splicing does not always follow the same fixed pattern (see alternative splicing ), the exact specification of exons is only possible to a limited extent, as different parts of a gene can be defined as exons depending on the finished mRNA. A precise prediction of exons using bioinformatics is therefore extremely difficult (see splice point and exon trapping ).

In addition, we now know proteins that are made up of exons from genes from spatially distant regions, sometimes even from different chromosomes . The traditional one-gene-one-enzyme hypothesis (also: one-gene-one-mRNA-one-protein hypothesis) is no longer tenable for higher organisms today.

Polycistronic mRNA consists of multiple ORFs in one transcript, with short regions of UTRs between the ORFs.

The regular sequence of exons and introns makes the typical structure of eukaryotic genes - the so-called interrupted gene ( English split gene ) - whose discovery Richard John Roberts and Phillip Allen Sharp with the 1993 Nobel Prize in Physiology or Medicine have been awarded.

The totality of the exons of an organism is called the exome .

history

The term exon was coined in 1978 by the biochemist Walter Gilbert : "The notion of the cistron ... 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. " The definition was originally developed for protein- introduced transcripts, but later for ribosomal RNA (rRNA), transfer RNA (tRNA) and trans-splicing ( English trans splicing extended).

Web links

Wiktionary: Exon  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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  3. ↑ International Human Genome Sequencing Consortium: Initial sequencing and analysis of the human genome. . In: Nature . 409, No. 6822, 2001, pp. 860-921. doi : 10.1038 / 35057062 . PMID 11237011 . [2]
  4. ↑ Kapranov, P. et al .: Examples of the complex architecture of the human transcriptome revealed by RACE and high-density tiling arrays . In: Genome Res . 15, No. 7, 2005, pp. 987-997. PMID 15998911 .
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  7. ^ Gilbert W: Why genes in pieces? . In: Nature . 271, No. 5645, 1978, p. 501. doi : 10.1038 / 271501a0 . PMID 622185 .
  8. ↑ Kister KP, Eckert WA: Characterization of an authentic intermediate in the self-splicing process of ribosomal precursor RNA in macronuclei of Tetrahymena thermophila . In: Nucleic Acids Research . 15, No. 5, March 1987, pp. 1905-1920. doi : 10.1093 / nar / 15.5.1905 . PMID 3645543 . PMC 340607 (free full text).
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  10. ↑ Liu AY, Van der Ploeg LH, Rijsewijk FA, Borst P: 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 . In: Journal of Molecular Biology . 167, No. 1, June 1983, pp. 57-75. doi : 10.1016 / S0022-2836 (83) 80034-5 . PMID 6306255 .