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Simple illustration of an unspliced mRNA precursor, with two introns and three exons (top). After the introns have been removed via splicing, the mature mRNA sequence is ready for translation (bottom). A particularly extreme case is the Drosophila dhc7 gene containing a ≥3.6 megabase (Mb) intron, which takes roughly three days to transcribe.
Some non-coding RNA transcripts also have exons and introns. Mature mRNAs originating from the same gene need not include the same exons, since different introns in the pre-mRNA can be removed by the process of alternative splicing. Exonization is the creation of a new exon, as a result of mutations in introns. [12]
The term intron refers to both the DNA sequence within a gene and the corresponding sequence in the unprocessed RNA transcript. As part of the RNA processing pathway, introns are removed by RNA splicing either shortly after or concurrent with transcription. [3] Introns are found in the genes of most organisms and many viruses.
The split gene theory is a theory of the origin of introns, long non-coding sequences in eukaryotic genes between the exons. [1] [2] [3] The theory holds that the randomness of primordial DNA sequences would only permit small (< 600bp) open reading frames (ORFs), and that important intron structures and regulatory sequences are derived from stop codons.
Introns are the parts of a gene that are transcribed into the precursor RNA sequence, but ultimately removed by RNA splicing during the processing to mature RNA. Introns are found in both types of genes: protein-coding genes and noncoding genes. They are present in prokaryotes but they are much more common in eukaryotic genomes. [citation needed]
In eukaryotic genes with multiple exons, introns are removed and exons are then joined together after transcription to yield the final mRNA for protein translation. In the context of gene finding , the start-stop definition of an ORF therefore only applies to spliced mRNAs , not genomic DNA, since introns may contain stop codons and/or cause ...
Alternative splicing produces three protein isoforms.Protein A includes all of the exons, whereas Proteins B and C result from exon skipping.. Alternative splicing, alternative RNA splicing, or differential splicing, is an alternative splicing process during gene expression that allows a single gene to produce different splice variants.
The introns, which make up a large part of the coding genes, are considered junk DNA. Junk DNA ( non-functional DNA ) is a DNA sequence that has no known biological function. [ 1 ] [ 2 ] Most organisms have some junk DNA in their genomes —mostly pseudogenes and fragments of transposons and viruses—but it is possible that some organisms have ...