An intron is any nucleotide sequence within a gene that is removed by RNA splicing during maturation of 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 sequence in RNA transcripts. Sequences that are joined together in the final mature RNA after RNA splicing are exons. Introns are found in the genes of most organisms and many viruses, and can be located in a wide range of genes, including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA). When proteins are generated from intron-containing genes, RNA splicing takes place as part of the RNA processing pathway that follows transcription and precedes translation.
RNA splicing, in molecular biology, is a form of RNA processing in which a newly made precursor messenger RNA (pre-mRNA) transcript is transformed into a mature messenger RNA (mRNA). Splicing occurs only in eukaryotic introns and introns often reside within sequences of protein-coding genes. During splicing, introns are removed and exons are joined together. For nuclear-encoded genes, splicing takes place within the nucleus either during or immediately after transcription. For those eukaryotic genes that contain introns, splicing is usually required in order to create an mRNA molecule that can be translated into protein. For many eukaryotic introns, splicing is carried out in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleo proteins (snRNPs). Self-splicing introns, or ribozymes capable of catalyzing their own excision from their parent RNA molecule, also exist.
Alternative splicing, or differential splicing, is a regulated process during gene expression that results in a single gene coding 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. Consequently, the proteins translated from alternatively spliced mRNAs will contain differences in their amino acid sequence and, often, in their biological functions. Notably, alternative splicing allows the human genome to direct the synthesis of many more proteins than would be expected from its 20,000 protein-coding genes.
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.
RNA-binding proteins are proteins that bind to the double or single stranded RNA in cells and participate in forming ribonucleoprotein complexes. RBPs contain various structural motifs, such as RNA recognition motif (RRM), dsRNA binding domain, zinc finger and others. They are cytoplasmic and nuclear proteins. However, since most mature RNA is exported from the nucleus relatively quickly, most RBPs in the nucleus exist as complexes of protein and pre-mRNA called heterogeneous ribonucleoprotein particles (hnRNPs). RBPs have crucial roles in various cellular processes such as: cellular function, transport and localization. They especially play a major role in post-transcriptional control of RNAs, such as: splicing, polyadenylation, mRNA stabilization, mRNA localization and translation. Eukaryotic cells encode diverse RBPs, approximately 500 genes, with unique RNA-binding activity and protein–protein interaction. During evolution, the diversity of RBPs greatly increased with the increase in the number of introns. Diversity enabled eukaryotic cells to utilize RNA exons in various arrangements, giving rise to a unique RNP (ribonucleoprotein) for each RNA. Although RBPs have a crucial role in post-transcriptional regulation in gene expression, relatively few RBPs have been studied systematically.
In genetics, a silencer is a DNA sequence capable of binding transcription regulation factors, called repressors. DNA contains genes and provides the template to produce messenger RNA (mRNA). That mRNA is then translated into proteins. When a repressor protein binds to the silencer region of DNA, RNA polymerase is prevented from transcribing the DNA sequence into RNA. With transcription blocked, the translation of RNA into proteins is impossible. Thus, silencers prevent genes from being expressed as proteins.
Cis-regulatory elements (CREs) are regions of non-coding DNA which regulate the transcription of neighboring genes. CREs are vital components of genetic regulatory networks, which in turn control morphogenesis, the development of anatomy, and other aspects of embryonic development, studied in evolutionary developmental biology.
Gene structure is the organisation of specialised sequence elements within a gene. Genes contain the information necessary for living cells to survive and reproduce. In most organisms, genes are made of DNA, where the particular DNA sequence determines the function of the gene. A gene is transcribed (copied) from DNA into RNA, which can either be non-coding (ncRNA) with a direct function, or an intermediate messenger (mRNA) that is then translated into protein. Each of these steps is controlled by specific sequence elements, or regions, within the gene. Every gene, therefore, requires multiple sequence elements to be functional. This includes the sequence that actually encodes the functional protein or ncRNA, as well as multiple regulatory sequence regions. These regions may be as short as a few base pairs, up to many thousands of base pairs long.
A splice site mutation is a genetic mutation that inserts, deletes or changes a number of nucleotides in the specific site at which splicing takes place during the processing of precursor messenger RNA into mature messenger RNA. Splice site consensus sequences that drive exon recognition are located at the very termini of introns. The deletion of the splicing site results in one or more introns remaining in mature mRNA and may lead to the production of abnormal proteins. When a splice site mutation occurs, the mRNA transcript possesses information from these introns that normally should not be included. Introns are supposed to be removed, while the exons are expressed. The mutation must occur at the specific site at which intron splicing occurs: within non-coding sites in a gene, directly next to the location of the exon. The mutation can be an insertion, deletion, frame shift, etc. The splicing process itself is controlled by the given sequences, known as splice-donor and splice-acceptor sequences, which surround each exon. Mutations in these sequences may lead to retention of large segments of intronic DNA by the mRNA, or to entire exons being spliced out of the mRNA. These changes could result in production of a nonfunctional protein. An intron is separated from its exon by means of the splice site. Acceptor-site and donor-site relating to the splice sites signal to the splicesome where the actual cut should be made. These donor sites, or recognition sites, are essential in the processing of mRNA. The average vertebrate gene consists of multiple small exons separated by introns that are considerably larger.
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.
Group II introns are a large class of self-catalytic ribozymes and mobile genetic elements found within the genes of all three domains of life. Ribozyme activity can occur under high-salt conditions in vitro. However, assistance from proteins is required for in vivo splicing. In contrast to group I introns, intron excision occurs in the absence of GTP and involves the formation of a lariat, with an A-residue branchpoint strongly resembling that found in lariats formed during splicing of nuclear pre-mRNA. It is hypothesized that pre-mRNA splicing may have evolved from group II introns, due to the similar catalytic mechanism as well as the structural similarity of the Domain V substructure to the U6/U2 extended snRNA. Finally, their ability to site-specifically mobilize to new DNA sites has been exploited as a tool for biotechnology.
In molecular biology, an exonic splicing enhancer (ESE) is a DNA sequence motif consisting of 6 bases within an exon that directs, or enhances, accurate splicing of heterogeneous nuclear RNA (hnRNA) or pre-mRNA into messenger RNA (mRNA).
An exonic splicing silencer (ESS) is a short region of an exon and is a cis-regulatory element. A set of 103 hexanucleotides known as FAS-hex3 has been shown to be abundant in ESS regions. ESSs inhibit or silence splicing of the pre-mRNA and contribute to constitutive and alternate splicing. To elicit the silencing affect, ESSs recruit proteins that will negatively affect the core splicing machinery.
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.
Glutamate receptor 3 is a protein that in humans is encoded by the GRIA3 gene.
Glutamate receptor 4 is a protein that in humans is encoded by the GRIA4 gene.
Bladder cancer-associated protein is a protein that in humans is encoded by the BLCAP gene.
Serine/arginine-rich splicing factor 1 (SRSF1) also known as alternative splicing factor 1 (ASF1), pre-mRNA-splicing factor SF2 (SF2) or ASF1/SF2 is a protein that in humans is encoded by the SRSF1 gene. ASF/SF2 is an essential sequence specific splicing factor involved in pre-mRNA splicing. SRSF1 is the gene that codes for ASF/SF2 and is found on chromosome 17. The resulting splicing factor is a protein of approximately 33 kDa. ASF/SF2 is necessary for all splicing reactions to occur, and influences splice site selection in a concentration-dependent manner, resulting in alternative splicing. In addition to being involved in the splicing process, ASF/SF2 also mediates post-splicing activities, such as mRNA nuclear export and translation.
Circular RNA is a type of RNA which, unlike the better known linear RNA, forms a covalently closed continuous loop, i.e., in circular RNA the 3' and 5' ends normally present in an RNA molecule have been joined together. This feature confers numerous properties to circular RNAs, many of which have only recently been identified.
A minigene is a minimal gene fragment that includes an exon and the control regions necessary for the gene to express itself in the same way as a wild type gene fragment. This is a minigene in its most basic sense. More complex minigenes can be constructed containing multiple exons and intron(s). Minigenes provide a valuable tool for researchers evaluating splicing patterns both in vivo and in vitro biochemically assessed experiments. Specifically, minigenes are used as splice reporter vectors and act as a probe to determine which factors are important in splicing outcomes. They can be constructed to test the way both cis-regulatory elements and trans-regulatory elements affect gene expression.
