NationMaster - Encyclopedia: Intron

Introns, derived from the term "Intervening Sequences", are sections of DNA colinear to the mRNA sequence that will be spliced out after transcription, but before the mRNA is translated. Introns are common in eukaryotic RNAs of all types, but are found in prokaryotic tRNA and rRNA genes only. The regions of a gene that remain in spliced mRNA are called exons. The number and length of introns varies widely among species and among genes within the same species. For example, the pufferfish Takifugu rubripes has little intronic DNA. Genes in mammals and flowering plants, on the other hand, often have numerous introns, which can be much longer than the nearby exons. Description: This image shows the coding region in a segment of eukaryotic DNA. Source: [1] (file) License: All of the illustrations in the Talking Glossary of Genetics are freely available and may be used without special permission. ... Description: This image shows the coding region in a segment of eukaryotic DNA. Source: [1] (file) License: All of the illustrations in the Talking Glossary of Genetics are freely available and may be used without special permission. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... In genetics, splicing is a modification of genetic information after transcription, in which introns of precursor messenger RNA (pre-mRNA) are removed and exons of it are joined. ... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ... The life cycle of an mRNA in a eukaryotic cell. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Chromalveolata Protista Alternative phylogeny Unikonta Opisthokonta Metazoa Choanozoa Eumycota Amoebozoa Bikonta Apusozoa Cabozoa Rhizaria Excavata Corticata Archaeplastida Chromalveolata Animals, plants, fungi, and protists are eukaryotes (IPA: ), organisms whose cells are organized into complex structures by internal membranes and a cytoskeleton. ... Prokaryotes (pro-KAR-ee-oht) (from Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl. ... Transfer RNA Transfer RNA (abbreviated tRNA), first hypothesized by Francis Crick, is a small RNA chain (73-93 nucleotides) that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. ... Ribosomal RNA (rRNA), a type of RNA synthesized in the nucleolus by RNA Pol I, is the central component of the ribosome, the protein manufacturing machinery of all living cells. ... An exon is any region of DNA within a gene, that is transcribed to the final messenger RNA (mRNA) molecule, rather than being spliced out from the transcribed RNA molecule. ... The hierarchy of scientific classification. ... Genera Amblyrhynchotes Arothron Auriglobus Canthigaster Carinotetraodon Chelonodon Colomesus Contusus Ephippion Feroxodon Fugu Gastrophysus Javichthys Lagocephalus Liosaccus Marilyna Monotretus Omegaphora Pelagocephalus Polyspina Reicheltia Sphoeroides Takifugu Tetractenos Tetraodon Torquigener Tylerius Xenopterus For species see Genera articles. ...

Introduction

Introns sometimes allow for alternative splicing of a gene, so that several different proteins that share some sections in common can be produced from a single gene. The control of mRNA splicing, and hence of which alternative is produced, is performed by a wide variety of signal molecules. Introns also sometimes contain "old code," sections of a gene that were probably once translated into protein but which are now discarded. Image File history File links Download high resolution version (1547x264, 11 KB) Summary Drawing of pre-mRNA to mRNA made by myself Licensing I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License... Image File history File links Download high resolution version (1547x264, 11 KB) Summary Drawing of pre-mRNA to mRNA made by myself Licensing I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License... Various modes of alternative splicing Alternative splicing is the process that occurs in eukaryotes in which the splicing process of a pre-mRNA transcribed from one gene can lead to different mature mRNA molecules and therefore to different proteins. ... The life cycle of an mRNA in a eukaryotic cell. ...

It was generally assumed that the sequence in any given intron is junk DNA with no function. More recently, this is being questioned however; it is known that introns contain several short sequences that are important for efficient splicing. The exact mechanism for these intronic splicing enhancers is not well understood, but it is thought that they serve as binding sites on the transcript for proteins that stabilize the spliceosome. It is also possible that RNA secondary structure formed by intronic sequences may have an effect on splicing, and in alternative splicing, an exonic sequence in one product is intronic in another. "Old code" sequences, on the other hand, in most cases indeed seem to be "evolutionary kipple". In molecular biology, junk DNA is a collective label for the portions of the DNA sequence of a chromosome or a genome for which no function has yet been identified. ... A spliceosome is a complex of RNA and many protein subunits called snRNPs, that removes the non-coding introns from unprocessed mRNA. Spliceosomes are unique to eukaryotic mRNA as the mRNA of prokaryotes lack introns. ... A representation of the 3D structure of the Myoglobin protein. ... Kipple is a term coined by science fiction author Philip K. Dick in the book Do Androids Dream of Electric Sheep?. It refers to unwanted or useless junk that tends to reproduce itself. ...

The discovery of introns led to the Nobel Prize in Physiology or Medicine in 1993 for Phillip Allen Sharp and Richard J. Roberts. The term intron was introduced by American biochemist Walter Gilbert in 1978: List of Nobel Prize laureates in Physiology or Medicine from 1901 to the present day. ... Phillip Allen Sharp (born 1944), U.S. geneticist and molecular biologist; co-discovered gene splicing; shared the 1993 Nobel Prize in Physiology or Medicine with Richard J. Roberts for the discovery that genes in eukaryotes are not contiguous strings but contain introns, and that the splicing of messenger RNA to... Richard J. Roberts (b. ... Biochemistry is the study of the chemical processes and transformations in living organisms. ... Walter Gilbert Walter Gilbert (born March 21, 1932) is an American physicist, biochemist,and molecular biology pioneer. ...

Some introns such as Group I and Group II introns are actually ribozymes that are capable of catalyzing their own splicing out of the primary RNA transcript. This self splicing was discovered by Thomas Cech who shared the 1989 Nobel Prize in Chemistry with Sidney Altman for the discovery of the catalytic properties of RNA.Intron also can be the regulator of gene expresion. // A ribozyme (from ribonucleic acid enzyme, also called RNA enzyme or catalytic RNA) is an RNA molecule that catalyzes a chemical reaction. ... In chemistry and biology, catalysis is the acceleration (increase in rate) of a chemical reaction by means of a substance, called a catalyst, that is itself not consumed by the overall reaction. ... Thomas R. Cech was born on December 8, 1947 in Chicago. ... This is a list of Nobel Prize laureates in Chemistry from 1901 to 2006. ... Sidney Altman Sidney Altman (born May 7, 1939) is a Canadian-born molecular biologist, who is currently the Sterling Professor of Molecular, Cellular, and Developmental Biology and Chemistry at Yale University. ...

Classification of introns

Sometimes group III introns are also identified as group II introns, because of their similarity in structure and function. In genetics, splicing is a modification of genetic information after transcription, in which introns of precursor messenger RNA (pre-mRNA) are removed and exons of it are joined. ... Group I catalytic introns are large self-splicing ribozymes. ... Group II intron is a class of intron found in rRNA, tRNA, mRNA of organelles in fungi, plants, protists, and mRNA in bacteria. ... Group III intron is a class of introns found in mRNA genes of chloroplasts in euglenoid protists. ...

Nuclear or spliceosomal introns are spliced by the spliceosome and a series of snRNAs (small nuclear RNAs). There are certain splice signals (or consensus sequences) which abet the splicing (or identification) of these introns by the spliceosome. In genetics, splicing is a modification of genetic information after transcription, in which introns of precursor messenger RNA (pre-mRNA) are removed and exons of it are joined. ... In genetics, splicing is a modification of genetic information after transcription, in which introns of precursor messenger RNA (pre-mRNA) are removed and exons of it are joined. ... A spliceosome is a complex of RNA and many protein subunits called snRNPs, that removes the non-coding introns from unprocessed mRNA. Spliceosomes are unique to eukaryotic mRNA as the mRNA of prokaryotes lack introns. ... Ribonucleic acid (RNA) is a nucleic acid consisting of a string of covalently-bound nucleotides. ... A spliceosome is a complex of RNA and many protein subunits called snRNPs, that removes the non-coding introns from unprocessed mRNA. Spliceosomes are unique to eukaryotic mRNA as the mRNA of prokaryotes lack introns. ...

Group I, II and III introns are self splicing introns and are relatively rare compared to spliceosomal introns. Group II and III introns are similar and have a conserved secondary structure. The lariat pathway is used in their splicing. They perform functions similar to the spliceosome and may be evolutionarily related to it. Group I introns are the only class of introns whose splicing requires a free guanine nucleoside. They possess a secondary structure different from that of group II and III introns. They are found in most bacteria and protozoa. In genetics, splicing is a modification of genetic information after transcription, in which introns of precursor messenger RNA (pre-mRNA) are removed and exons of it are joined. ... A representation of the 3D structure of the Myoglobin protein. ... This article is about evolution in biology. ... Guanine is one of the five main nucleobases found in the nucleic acids DNA and RNA; the others being adenine, cytosine, thymine, and uracil. ... Nucleosides are glycosylamines made by attaching a nucleobase (often reffered to simply as bases) to a ribose ring. ...

Intron evolution

There are two competing theories attempting to explain the origin and evolution of spliceosomal introns (Other classes of introns such as self-splicing and tRNA introns are not subject to much debate, but see ^[1] for the former). These are popularly called as the Introns-Early (IE) or the Introns-Late (IL) views. This article is about evolution in biology. ... A spliceosome is a complex of RNA and many protein subunits called snRNPs, that removes the non-coding introns from unprocessed mRNA. Spliceosomes are unique to eukaryotic mRNA as the mRNA of prokaryotes lack introns. ... Transfer RNA (abbreviated tRNA) is a small RNA chain (74-93 nucleotides) that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. ...

The IE model, championed by Walter Gilbert^[2], proposes that introns are extremely old and numerously present in the earliest ancestors of prokaryotes and eukaryotes (the progenote). In this model introns were subsequently lost from prokaryotic organisms, allowing them to attain growth efficiency. A central prediction of this theory is that the early introns were mediators that facilitated the recombination of exons that represented the protein domains. Such a model would directly lead to the evolution of new genes. Prokaryotes are unicellular (in rare cases, multicellular) organisms without a nucleus. ... Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... An exon is any region of DNA within a gene, that is transcribed to the final messenger RNA (mRNA) molecule, rather than being spliced out from the transcribed RNA molecule. ...

The IL model proposes that introns were more recently inserted into original intron-less contiguous genes after the divergence of eukaryotes and prokaryotes. In this model, introns probably had their origin in parasitic transposable elements. This model is based on the observation that the spliceosomal introns are restricted to eukaryotes alone. However, there is considerable debate on the presence of introns in the early prokaryote-eukaryote ancestors and the subsequent intron loss-gain during eukaryotic evolution. It is also suggested that the evolution of introns and more generally the intron-exon structure is largely independent of the coding-sequence evolution^[3]. Transposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process called Transposition. ...

Identification

Nearly all eukaryotic nuclear introns begin with GU and end with AG (the GU-AG rule). This mainly occurs in eukaryotic mRNA. Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ...

See also

Splicing: An exon is any region of DNA within a gene, that is transcribed to the final messenger RNA (mRNA) molecule, rather than being spliced out from the transcribed RNA molecule. ... In genetics, splicing is a modification of genetic information after transcription, in which introns are removed and exons are joined. ... Twintrons are introns-within-introns excised by sequential splicing reactions. ...

Others: Various modes of alternative splicing Alternative splicing is the process that occurs in eukaryotes in which the splicing process of a pre_mRNA can lead to different ripe mRNA molecules and therefore to different proteins. ... The minor spliceosome is a ribonucleoprotein complex that catalyses the removal of an atypical class of spliceosomal introns (U12-type) from eukaryotic messenger RNAs in plant, insects, vertebrates and some fungi (Rhizopus oryzae). ...

Eukaryotic chromosome fine structure refers to the structure of sequences for eukaryotic chromosomes. ... 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. ... It has been suggested that junk DNA be merged into this article or section. ... Selfish DNA is DNA which is prevalent in the genome, not because its phenotypic effect is beneficial, but because it has properties which cause the number of copies of it within the genome to increase with time. ...

References

^ Gogarten JP, Hilario E. "Inteins, introns, and homing endonucleases: recent revelations about the life cycle of parasitic genetic elements." BMC Evol Biol. 2006 Nov 13; 6: 94. PMID 17101053
^ Fedorov A, Cao X, Saxonov S, de Souza SJ, Roy SW, Gilbert W. "Intron distribution difference for 276 ancient and 131 modern genes suggests the existence of ancient introns." Proc Natl Acad Sci U S A. 2001 Nov 6; 98(23): 13177-82. PMID 11687643.
^ Yandell M, Mungall CJ, Smith C, Prochnik S, Kaminker J, Hartzell G, Lewis S, Rubin GM. "Large-scale trends in the evolution of gene structures within 11 animal genomes." PLoS Comput Biol. 2006 Mar; 2(3): e15. PMID 16518452

Walter Gilbert Walter Gilbert (born March 21, 1932) is an American physicist, biochemist,and molecular biology pioneer. ... Nature is one of the most prominent scientific journals, first published on 4 November 1869. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A cover of Nature Reviews Genetics Nature Reviews Genetics is a monthly review journal in genetics and covers the full breadth of modern genetics. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... BioMed Central (BMC) is a UK-based scientific publisher specializing in open access publication. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... PLoS Computational Biology is an open-access computational biology journal published by the nonprofit organization Public Library of Science in association with the International Society for Computational Biology (ISCB). ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

External links

Categories: Gene expression | DNA To meet Wikipedias quality standards, this article or section may require cleanup. ... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ... The 5 cap is a specially altered dinucleotide end to the 5 end of preliminary messenger RNA as found in eukaryotes. ... In genetics, splicing is a modification of genetic information after transcription, in which introns of precursor messenger RNA (pre-mRNA) are removed and exons of it are joined. ... Precursor mRNA, more commonly termed pre-mRNA, is an incompletely processed single strand of messenger ribonucleic acid (mRNA), synthesized from a DNA template in the nucleus of a cell by transcription. ... An exon is any region of DNA within a gene, that is transcribed to the final messenger RNA (mRNA) molecule, rather than being spliced out from the transcribed RNA molecule. ... SnRNPs (pronounced snurps), or small nuclear ribonucleoproteins, are particles which combine with pre-mRNA and various proteins to form spliceosomes (a type of large molecular complex). ... A spliceosome is a complex of RNA and many protein subunits called snRNPs, that removes the non-coding introns from unprocessed mRNA. Spliceosomes are unique to eukaryotic mRNA as the mRNA of prokaryotes lack introns. ... Various modes of alternative splicing Alternative splicing is the process that occurs in eukaryotes in which the splicing process of a pre-mRNA transcribed from one gene can lead to different mature mRNA molecules and therefore to different proteins. ... Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA (mRNA) molecule. ...

Contents