In molecular biology, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. Molecular biology is the study of Biology at a molecular level Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units Transcription is the synthesis of RNA under the direction of DNA Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into An exon is a Nucleic acid sequence that is represented in the mature form of an RNA molecule after a portions of a precursor RNA Introns have been This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product Translation is the first stage of Protein biosynthesis (part of the overall process of Gene expression) For many eukaryotic introns, splicing is done in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs), but there are also self-splicing introns. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst A spliceosome is a complex of specialized RNA and Protein subunits that removes Introns from a transcribed pre- mRNA ( HnRNA snRNP s (pronounced "snurps" or small nuclear ribonucleoproteins, are particles that combine with Pre-mRNA and various proteins to form Spliceosomes
Several methods of RNA splicing occur in nature: the type of splicing depends on the structure of the spliced intron and the catalysts required for splicing to occur. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst
Spliceosomal introns often reside in eukaryotic protein-coding genes. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Within the intron, a 3' splice site, 5' splice site, and branch site are required for splicing. Splicing is catalyzed by the spliceosome which is a large RNA-protein complex composed of five small nuclear ribonucleoproteins (snRNPs, pronounced 'snurps' ). A spliceosome is a complex of specialized RNA and Protein subunits that removes Introns from a transcribed pre- mRNA ( HnRNA snRNP s (pronounced "snurps" or small nuclear ribonucleoproteins, are particles that combine with Pre-mRNA and various proteins to form Spliceosomes The RNA components of snRNPs interact with the intron and may be involved in catalysis. Two types of spliceosomes have been identified (the major and minor) which contain different snRNPs. snRNP s (pronounced "snurps" or small nuclear ribonucleoproteins, are particles that combine with Pre-mRNA and various proteins to form Spliceosomes
Self-splicing occurs for rare introns that form a ribozyme, performing the functions of the spliceosome by RNA alone. A ribozyme (from ribo nucleic acid en' zyme', also called RNA Enzyme or catalytic RNA is an RNA Molecule that catalyzes There are three kinds of self-splicing introns, Group I, Group II and Group III. Group I catalytic introns are large self-splicing Ribozymes. They catalyze their own excision from MRNA, TRNA and RRNA precursors Group II intron is a class of Intron found in RRNA, TRNA, MRNA of organelles in Fungi, plants Protists and mRNA in Group III intron is a class of Introns found in mRNA genes of chloroplasts in euglenoid Protists They have a conventional group II-type dVI with a bulged Group I and II introns perform splicing similar to the spliceosome without requiring any protein. This similarity suggests that Group I and II introns may be evolutionarily related to the spliceosome. Self-splicing may also be very ancient, and may have existed in an RNA world that was present before protein. The RNA world hypothesis proposes that a world filled with life based on Ribonucleic acid (RNA predated current life based on Deoxyribonucleic acid (DNA Although the two splicing mechanisms described below do not require any proteins to occur, 5 additional RNA molecules and over 50 proteins are used and hydrolyzes many ATP molecules. The splicing mechanisms use ATP in order to accurately splice mRNA's. If the cell were to not use any ATP's, the process would be highly inaccurate and many mistakes would occur. Two transesterifications characterize the mechanism in which group I introns are sliced: 1) 3'OH of a free guanine nucleoside (or one located in the intron) or a nucleotide cofactor (GMP, GDP, GTP) attacks phosphate at the 5' splice site. 2) 3'OH of the 5'exon becomes a nucleophile and the second transesterification results in the joining of the two exons. The mechanism in which group II introns are spliced (two transesterification reaction like group I introns) is as follows: 1)The 2'OH of a specific adenosine in the intron attacks the 5' splice site, thereby forming the lariat 2) The 3'OH of the 5' exon triggers the second transesterification at the 3' splice site thereby joining the exons together.
tRNA (also tRNA-like) splicing is another rare form of splicing that usually occurs in tRNA. Transfer RNA (abbreviated tRNA) is a small RNA (usually about 74-95 nucleotides that transfers a specific Amino acid to a growing polypeptide chain at The splicing reaction involves a different biochemistry than the spliceomsomal and self-splicing pathways. Ribonucleases cleave the RNA and ligases join the exons together. Ribonuclease, abbreviated commonly as RNase, is a Nuclease that catalyzes the degradation of RNA into smaller components In Biochemistry, a ligase (from the Latin verb ligāre &mdash "to bind" or "to glue together" is an Enzyme that can catalyse
Splicing occurs in all the kingdoms or domains of life, however, the extent and types of splicing can be very different between the major divisions. In biological Taxonomy, a kingdom or regnum is a Taxonomic rank in either (historically the highest rank or (in the new three-domain system In biological Taxonomy, a domain (also superregnum, superkingdom, or empire) is the highest Taxonomic rank of Organisms Eukaryotes splice many protein-coding messenger RNAs and some non-coding RNAs. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product A non-coding RNA ( ncRNA) is any RNA molecule that is not translated into a Protein. Prokaryotes, on the other hand, splice rarely, but mostly non-coding RNAs. The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other Another important difference between these two groups of organisms is that prokaryotes completely lack the spliceosomal pathway.
Because spliceosomal introns are not conserved in all species, there is debate concerning when spliceosomal splicing evolved. Two models have been proposed: the intron late and intron early models (see intron evolution). Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into
Spliceosomal splicing and self-splicing involves a two-step biochemical process. Both steps involve transesterification reactions that occur between RNA nucleotides. In Organic chemistry, transesterification is the process of exchanging the Alcohol group of an Ester compound with another Alcohol. tRNA splicing, however, is an exception and does not occur by transesterification.
Spliceosomal and self-splicing transesterification reactions occur via two sequential transesterification reactions. First, the 2'OH of a specific branch-point nucleotide within the intron that is defined during spliceosome assembly performs a nucleophilic attack on the first nucleotide of the intron at the 5' splice site forming the lariat intermediate. In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to Second, the 3'OH of the released 5' exon then performs a nucleophilic attack at the last nucleotide of the intron at the 3' splice site thus joining the exons and releasing the intron lariat.
In many cases, the splicing process can create a range of unique proteins by varying the exon composition of the same messenger RNA. Alternative splicing is the RNA splicing variation mechanism in which the Exons of the primary gene transcript the Pre-mRNA, are separated and reconnected This phenomenon is then called alternative splicing. Alternative splicing is the RNA splicing variation mechanism in which the Exons of the primary gene transcript the Pre-mRNA, are separated and reconnected
Splicing events can be experimentally altered by binding steric-blocking antisense oligos such as Morpholinos or Peptide nucleic acids to snRNP binding sites, to the branchpoint nucleotide that closes the lariat, or to splice-regulatory element binding sites. In Molecular biology, a Morpholino is a Molecule used to modify Gene expression. Peptide nucleic acid (PNA is an artificially synthesized Polymer similar to DNA or RNA and is used in biological research and medical treatments 
Not only pre-mRNA but also proteins can undergo splicing. Protein splicing is an intramolecular reaction of a particular Protein in which an internal protein segment (called an Intein) is removed from a precursor protein Although the biomolecular mechanisms are different, the principle is the same, that parts of the protein, called inteins instead of introns, are removed. An intein is a segment of a Protein that is able to excise itself and rejoin the remaining portions (the exteins with a Peptide bond. The remaining parts, called exteins instead of exons, are fused together. Protein splicing has been observed in lower organisms, yeast, plants and animals, including in humans.