An intron is any nucleotide sequence within a gene that is removed by RNA splicing during maturation of the final RNA product. In other words, introns are non-coding regions of an RNA transcript, or the DNA encoding it, that are eliminated by splicing before translation. 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 sequence in RNA transcripts. Sequences that are joined in the final mature RNA after RNA splicing are 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 introns and so joining 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.
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.
snRNPs, or small nuclear ribonucleoproteins, are RNA-protein complexes that combine with unmodified pre-mRNA and various other proteins to form a spliceosome, a large RNA-protein molecular complex upon which splicing of pre-mRNA occurs. The action of snRNPs is essential to the removal of introns from pre-mRNA, a critical aspect of post-transcriptional modification of RNA, occurring only in the nucleus of eukaryotic cells. Additionally, U7 snRNP is not involved in splicing at all, as U7 snRNP is responsible for processing the 3′ stem-loop of histone pre-mRNA.
Small nuclear RNA (snRNA) is a class of small RNA molecules that are found within the splicing speckles and Cajal bodies of the cell nucleus in eukaryotic cells. The length of an average snRNA is approximately 150 nucleotides. They are transcribed by either RNA polymerase II or RNA polymerase III. Their primary function is in the processing of pre-messenger RNA (hnRNA) in the nucleus. They have also been shown to aid in the regulation of transcription factors or RNA polymerase II, and maintaining the telomeres.
The minor spliceosome is a ribonucleoprotein complex that catalyses the removal (splicing) of an atypical class of spliceosomal introns (U12-type) from eukaryotic messenger RNAs in plants, insects, vertebrates and some fungi. 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.
The U11 snRNA is an important non-coding RNA in the minor spliceosome protein complex, which activates the alternative splicing mechanism. The minor spliceosome is associated with similar protein components as the major spliceosome. It uses U11 snRNA to recognize the 5' splice site while U12 snRNA binds to the branchpoint to recognize the 3' splice site.
U12 minor spliceosomal RNA is formed from U12 small nuclear (snRNA), together with U4atac/U6atac, U5, and U11 snRNAs and associated proteins, forms a spliceosome that cleaves a divergent class of low-abundance pre-mRNA introns. Although the U12 sequence is very divergent from that of U2, the two are functionally analogous.
U2 spliceosomal snRNAs are a species of small nuclear RNA (snRNA) molecules found in the major spliceosomal (Sm) machinery of virtually all eukaryotic organisms. In vivo, U2 snRNA along with its associated polypeptides assemble to produce the U2 small nuclear ribonucleoprotein (snRNP), an essential component of the major spliceosomal complex. The major spliceosomal-splicing pathway is occasionally referred to as U2 dependent, based on a class of Sm intron—found in mRNA primary transcripts—that are recognized exclusively by the U2 snRNP during early stages of spliceosomal assembly. In addition to U2 dependent intron recognition, U2 snRNA has been theorized to serve a catalytic role in the chemistry of pre-RNA splicing as well. Similar to ribosomal RNAs (rRNAs), Sm snRNAs must mediate both RNA:RNA and RNA:protein contacts and hence have evolved specialized, highly conserved, primary and secondary structural elements to facilitate these types of interactions.
U5 snRNA is a small nuclear RNA (snRNA) that participates in RNA splicing as a component of the spliceosome. It forms the U5 snRNP by associating with several proteins including Prp8 - the largest and most conserved protein in the spliceosome, Brr2 - a helicase required for spliceosome activation, Snu114, and the 7 Sm proteins. U5 snRNA forms a coaxially-stacked series of helices that project into the active site of the spliceosome. Loop 1, which caps this series of helices, forms 4-5 base pairs with the 5'-exon during the two chemical reactions of splicing. This interaction appears to be especially important during step two of splicing, exon ligation.
U6 snRNA is the non-coding small nuclear RNA (snRNA) component of U6 snRNP, an RNA-protein complex that combines with other snRNPs, unmodified pre-mRNA, and various other proteins to assemble a spliceosome, a large RNA-protein molecular complex that catalyzes the excision of introns from pre-mRNA. Splicing, or the removal of introns, is a major aspect of post-transcriptional modification and takes place only in the nucleus of eukaryotes.
U4atac minor spliceosomal RNA is a ncRNA which is an essential component of the minor U12-type spliceosome complex. The U12-type spliceosome is required for removal of the rarer class of eukaryotic introns.
Splicing factor U2AF 65 kDa subunit is a protein that in humans is encoded by the U2AF2 gene.
RNA-binding protein 8A is a protein that in humans is encoded by the RBM8A gene.
Pre-mRNA-processing-splicing factor 8 is a protein that in humans is encoded by the PRPF8 gene.
Splicing factor 1 also known as zinc finger protein 162 (ZFM162) is a protein that in humans is encoded by the SF1 gene.
Splicing factor 3B subunit 1 is a protein that in humans is encoded by the SF3B1 gene.
U4/U6 small nuclear ribonucleoprotein Prp4 is a protein that in humans is encoded by the PRPF4 gene. The removal of introns from nuclear pre-mRNAs occurs on complexes called spliceosomes, which are made up of 4 small nuclear ribonucleoprotein (snRNP) particles and an undefined number of transiently associated splicing factors. PRPF4 is 1 of several proteins that associate with U4 and U6 snRNPs.[supplied by OMIM]
RNA, U2 small nuclear, also known as RNU2, is a human gene.
Prp8 refers to both the Prp8 protein and Prp8 gene. Prp8's name originates from its involvement in pre-mRNA processing. The Prp8 protein is a large, highly conserved, and unique protein that resides in the catalytic core of the spliceosome and has been found to have a central role in molecular rearrangements that occur there. Prp8 protein is a major central component of the catalytic core in the spliceosome, and the spliceosome is responsible for splicing of precursor mRNA that contains introns and exons. Unexpressed introns are removed by the spliceosome complex in order to create a more concise mRNA transcript. Splicing is just one of many different post-transcriptional modifications that mRNA must undergo before translation. Prp8 has also been hypothesized to be a cofactor in RNA catalysis.
