What are introns and exons in DNA
Introns (Intervening regions) are the non-coding sections of DNA within a gene (intragen) that separate neighboring exons. Introns are transcribed but then spliced out of the pre-mRNA before it is pushed out of the nucleus for translation. 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", i.e. (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.
Introns play a role in alternative splicingalternative 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 virtually remove themselves from the mRNA, play a special role.
The ratio of intronic to exonic DNA varies greatly between different species. The puffer fish Fugu rubripes for example, because of its very low proportion of introns, it was sequenced early on, even compared to related species.
One can consider the introns as a subset of the so-called junk DNA Consider ("DNA junk"), which is the total of all non-coding DNA fractions. They lie outside the genes to which no function could be assigned, but which may play a role in gene regulation and in the regulation of alternative splicing.
The "mosaic genes", in which coding DNA regions (exon) are separated by non-coding (intron) DNA regions, were only discovered in 1977 by Hogness, Mandel and Chambon.
Introns can be located at virtually 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 Phase 1 introns and between the second and third base of one Phase 2 intron. This is important when there is duplication of exon. An exon that lies between two introns of the same phase (called a "symmetrical exon") can easily be duplicated without causing a grid shift. Asymmetrical exons that lie between two introns of different phases cannot be duplicated.
Types of introns
Depending on whether the splicing process takes place autonomously or 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 by Thomas R. Cech in 1981, 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 exon during splicing depends on the sequence.
The most common introns are the so-called GT-AG introns. They usually start with a GT (guanine-thymine) and end with an AG. (Adenine-guanine)
In 2007, introns were found in 36 human genes, which unusually end in TG (thymine-guanine) at the right end. It is currently assumed that this will delay the splicing of the mRNA, which should allow it to mature again, and further research is expected Therapeutic approaches against hereditary diseases and cancer. See []
AT-AC introns, on the other hand, begin with an AT (adenine-thymine) and end with an AC (adenine-cytosine).
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