Introns ( English Intr agenic regi ons ), the non-coding portions of the DNA within a gene (intragenic), adjacent exons separated. Introns are transcribed , but then from the pre-mRNA spliced out before them for translation from the nucleus is removed funneled. The parts of the gene that remain in the mature mRNA are called exons. The division of the gene into introns and exons is one of the main characteristics of eukaryotic cells .
Introns can contain “old code”, ie (duplicated) parts of a gene that have become functionless in the course of the tribal history . Since they have no direct significance for the structure of the translation products, they tend to accumulate mutations to a greater extent than exons. In higher eukaryotes they take up a considerable part of the genome. Some assume that they are leftovers from viral infections or "defective" endogenous (retro) viruses. In humans, these components make up at least 50% of the genome.
Introns play a role in the alternative splicing of a gene, so that a gene can produce several proteins that differ in sections. In these cases, the splicing process decides whether a DNA sequence is treated as an intron or an exon .
The self-splicing introns ( ribozymes ), which remove themselves from the mRNA, play a special role.
The ratio of intronic to exonic DNA varies greatly between different species. The pufferfish Takifugu rubripes, for example, was sequenced at an early stage due to its very low proportion of introns, even compared to related species .
The introns can be viewed as a subset of the so-called noncoding DNA , which is the total amount of all non-coding DNA components.
The mosaic genes , in which coding DNA regions (exon) are separated by non-coding (intron) DNA regions, were independently recognized in 1977 by Richard J. Roberts and Phillip Allen Sharp , for which they received the 1993 Nobel Prize in Medicine .
The term intron 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 introduced for protein-coding transcripts, but was later expanded to include rRNA , tRNA and trans splicing .
Introns can be located at almost any point in the transcript, even in the middle of a block of three that functions as a codon in translation. If an intron lies between the third base of a codon and the first of the next codon (i.e. between two codons), one speaks of phase 0 introns . If the intron lies between the first and the second nucleotide of a codon, one speaks of a phase 1 intron and a phase 2 intron between the second and third base . This is important when there is duplication of exons . An exon that lies between two introns of the same phase (called a “symmetrical exon”) can be duplicated without any problems without causing a frame shift . Asymmetrical exons that lie between two introns of different phases cannot be duplicated.
Types of introns
Depending on whether the splicing process is autonomous or occurs through a riboprotein complex (the spliceosome ), a distinction is made between self-splicing and spliceosomal introns.
In the case of the self-splicing introns, which were discovered in 1981 by Thomas R. Cech's group , a distinction is made again:
- Group I introns
- Group II introns
For more details see under splicing .
These introns must be cut out through the spliceosome. Whether a sequence is recognized as an intron or an exon during splicing depends on the sequence.
In 2007, introns were found in 36 human genes which, unusually, end in UG ( uracil - guanine ) at the right end . So far, it has been assumed that this will delay the splicing of the mRNA, which should allow it to mature again, and new therapeutic approaches against hereditary diseases and cancer are expected from further research.
Introns in science fiction
In the episode Genesis (7x19) of the science fiction series Spaceship Enterprise: The Next Century , introns are presented as genetic evolutionary “remnants” that carry the genetic information of archetypes of species. Due to an accident, the spaceship crew developed back to these archetypes.
Also in the series Outer Limits - The Unknown Dimension , introns are a topic in the episode Genetic Message (3x12). A scientist carries out gene experiments and a self-experiment there. This leads to extraordinary changes in his body, especially the brain.
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