Exon

As exon (of English. Overexpressed region) the part of a eukaryotic gene is referred to, which is retained after splicing ( splicing ). This contrasts with the introns (of English. Intragenic regions) that are cut out and removed during splicing. The typical human gene containing an average of eight exons with an average length of 145 nucleotides of the internal exons. Introns are more than 10 times as long, on average, in some cases they are even much longer.

Exons protein encoding genes contain the open reading frame (ORF ) plus the 5 'and 3' untranslated region (UTR ) from the terminal exon. Only about 1.5 percent of the total genomic DNA coding for proteins ( the haploid human genome amounts to approximately 23,000 protein - coding genes), while the rest of genes for non-coding RNA, as well as introns, regulatory and noncoding DNA deoxyribonucleic acids ( so-called "junk " DNA) exists. Since many of the protein-coding genes, including through alternative splicing of the primary transcript ( precursor mRNA) of a gene produce more than one protein, come in the human body but still far more than 23,000 different proteins. This is not decided until during the splicing which DNA sequences are introns and exons that. In other words, is spliced ​​Since not always follow the same fixed pattern (see Alternative splicing ), the exact specification of exons is only possible because, depending on the finished mRNA different parts of a gene can be defined as exons. An accurate prediction of exons by means of bioinformatics is therefore extremely difficult ( cf. splice site and exon trapping ).

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

Polycistronic mRNA consists of multiple ORFs in a 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 Mosaikgen (English split gene) -, whose discovery by Richard John Roberts and Phillip A. Sharp was awarded the 1993 Nobel Prize for Medicine.

History

The term exon was coined in 1978 by biochemist Walter Gilbert: "The notion of the cistron ... TO BE REPLACED did of a transcription unit Containing regions whichwill be lost from the mature messenger - Which I suggest we call introns ( for intragenic regions) - alternating with regions whichwill be overexpressed - exons ". The definition was originally introduced for protein-coding transcripts, but later for rRNA, tRNA and trans- splicing expands.