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An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term exon refers to both the DNA sequence within a gene and to the corresponding sequence in RNA transcripts. In RNA splicing, introns are removed and exons are covalently joined to one another as part of generating the mature RNA. Just as the entire set of genes for a species constitutes the genome, the entire set of exons constitutes the exome.
An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word intron is derived from the term intragenic region, i.e., a region inside a gene. The term intron refers to both the DNA sequence within a gene and the corresponding RNA sequence in RNA transcripts. The non-intron sequences that become joined by this RNA processing to form the mature RNA are called exons.
RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre-mRNA) transcript is transformed into a mature messenger RNA (mRNA). It works by removing all the introns and splicing back together exons. For nuclear-encoded genes, splicing occurs in the nucleus either during or immediately after transcription. For those eukaryotic genes that contain introns, splicing is usually needed to create an mRNA molecule that can be translated into protein. For many eukaryotic introns, splicing occurs in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). There exist self-splicing introns, that is, ribozymes that can catalyze their own excision from their parent RNA molecule. The process of transcription, splicing and translation is called gene expression, the central dogma of molecular biology.
Alternative splicing, or alternative RNA splicing, or differential splicing, is an alternative splicing process during gene expression that allows a single gene to code for multiple proteins. In this process, particular exons of a gene may be included within or excluded from the final, processed messenger RNA (mRNA) produced from that gene. This means the exons are joined in different combinations, leading to different (alternative) mRNA strands. Consequently, the proteins translated from alternatively spliced mRNAs usually contain differences in their amino acid sequence and, often, in their biological functions.
A spliceosome is a large ribonucleoprotein (RNP) complex found primarily within the nucleus of eukaryotic cells. The spliceosome is assembled from small nuclear RNAs (snRNA) and numerous proteins. Small nuclear RNA (snRNA) molecules bind to specific proteins to form a small nuclear ribonucleoprotein complex, which in turn combines with other snRNPs to form a large ribonucleoprotein complex called a spliceosome. The spliceosome removes introns from a transcribed pre-mRNA, a type of primary transcript. This process is generally referred to as splicing. An analogy is a film editor, who selectively cuts out irrelevant or incorrect material from the initial film and sends the cleaned-up version to the director for the final cut.
A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have unique functions. A set of protein isoforms may be formed from alternative splicings, variable promoter usage, or other post-transcriptional modifications of a single gene; post-translational modifications are generally not considered. Through RNA splicing mechanisms, mRNA has the ability to select different protein-coding segments (exons) of a gene, or even different parts of exons from RNA to form different mRNA sequences. Each unique sequence produces a specific form of a protein.
A primary transcript is the single-stranded ribonucleic acid (RNA) product synthesized by transcription of DNA, and processed to yield various mature RNA products such as mRNAs, tRNAs, and rRNAs. The primary transcripts designated to be mRNAs are modified in preparation for translation. For example, a precursor mRNA (pre-mRNA) is a type of primary transcript that becomes a messenger RNA (mRNA) after processing.
IKBKAP is a human gene encoding the IKAP protein, which is ubiquitously expressed at varying levels in all tissue types, including brain cells. The IKAP protein is thought to participate as a sub-unit in the assembly of a six-protein putative human holo-Elongator complex, which allows for transcriptional elongation by RNA polymerase II. Further evidence has implicated the IKAP protein as being critical in neuronal development, and directs that decreased expression of IKAP in certain cell types is the molecular basis for the severe, neurodevelopmental disorder familial dysautonomia. Other pathways that have been connected to IKAP protein function in a variety of organisms include tRNA modification, cell motility, and cytosolic stress signalling. Homologs of the IKBKAP gene have been identified in multiple other Eukaryotic model organisms. Notable homologs include Elp1 in yeast, Ikbkap in mice, and D-elp1 in fruit flies. The fruit fly homolog (D-elp1) has RNA-dependent RNA polymerase activity and is involved in RNA interference.
Heterogeneous nuclear ribonucleoproteins (hnRNPs) are complexes of RNA and protein present in the cell nucleus during gene transcription and subsequent post-transcriptional modification of the newly synthesized RNA (pre-mRNA). The presence of the proteins bound to a pre-mRNA molecule serves as a signal that the pre-mRNA is not yet fully processed and therefore not ready for export to the cytoplasm. Since most mature RNA is exported from the nucleus relatively quickly, most RNA-binding protein in the nucleus exist as heterogeneous ribonucleoprotein particles. After splicing has occurred, the proteins remain bound to spliced introns and target them for degradation.
The minor spliceosome is a ribonucleoprotein complex that catalyses the removal (splicing) of an atypical class of spliceosomal introns (U12-type) from messenger RNAs in some clades of eukaryotes. This process is called noncanonical splicing, as opposed to U2-dependent canonical splicing. U12-type introns represent less than 1% of all introns in human cells. However they are found in genes performing essential cellular functions.
Splicing factor U2AF 65 kDa subunit is a protein that in humans is encoded by the U2AF2 gene.
Pre-mRNA-processing-splicing factor 8 is a protein that in humans is encoded by the PRPF8 gene.
Splicing factor U2AF 35 kDa subunit is a protein that in humans is encoded by the U2AF1 gene.
SON protein is a protein that in humans is encoded by the SON gene.
Pre-mRNA-processing factor 6 is a protein that in humans is encoded by the PRPF6 gene.
Pre-mRNA-processing factor 17 is a protein that in humans is encoded by the CDC40 gene.
Polypyrimidine tract-binding protein 1 is a protein that in humans is encoded by the PTBP1 gene.
Pre-mRNA-splicing factor SLU7 is a protein that in humans is encoded by the SLU7 gene.
Rev is a transactivating protein that is essential to the regulation of HIV-1 protein expression. A nuclear localization signal is encoded in the rev gene, which allows the Rev protein to be localized to the nucleus, where it is involved in the export of unspliced and incompletely spliced mRNAs. In the absence of Rev, mRNAs of the HIV-1 late (structural) genes are retained in the nucleus, preventing their translation.
Prp24 is a protein part of the pre-messenger RNA splicing process and aids the binding of U6 snRNA to U4 snRNA during the formation of spliceosomes. Found in eukaryotes from yeast to E. coli, fungi, and humans, Prp24 was initially discovered to be an important element of RNA splicing in 1989. Mutations in Prp24 were later discovered in 1991 to suppress mutations in U4 that resulted in cold-sensitive strains of yeast, indicating its involvement in the reformation of the U4/U6 duplex after the catalytic steps of splicing.
References
- ↑ "Q9Y312 · AAR2_HUMAN". www.uniprot.org. UniProt. Retrieved 2023-12-14.
- ↑ Nakazawa, N.; Harashima, S.; Oshima, Y. (November 1991). "AAR2, a gene for splicing pre-mRNA of the MATa1 cistron in cell type control of Saccharomyces cerevisiae". Molecular and Cellular Biology. 11 (11): 5693–5700. doi:10.1128/mcb.11.11.5693-5700.1991. ISSN 0270-7306. PMC 361940 . PMID 1922071.
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