Intron


Intron

Introns, derived from the term "intragenic regions" and also called "intervening sequence" (IVS) [Cite web
url = http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-I/intron.html
title = intron (intervening sequence)
author = Mark Lefers
publisher = Department of Biochemistry, Molecular Biology, and Cell Biology, Weinberg College of Arts & Sciences, Northwestern University
accessdate = 2008-06-17
] , are DNA regions in a gene that are not translated into proteins. These non-coding sections are present in precursor mRNA (pre-mRNA) and some other RNAs, and removed by a process called splicing during the processing to mature RNA. After intron splicing, the mRNA consists only of exons, which are translated into a protein.

Introduction

Introns are common in eukaryotic pre-mRNA, but in prokaryotes they are only found in tRNA and rRNA; introns have variable length and alternate with exons in intron-containing genes.

The number and length of introns varies widely among species, and among genes within the same species. Some eukaryotes, e.g. sac fungi, have evolved genomes with few introns, [cite journal |author=Stajich JE, Dietrich FS, Roy SW |title=Comparative genomic analysis of fungal genomes reveals intron-rich ancestors |journal=Genome Biol. |volume=8 |issue=10 |pages=R223 |year=2007 |pmid=17949488 |pmc=2246297 |doi=10.1186/gb-2007-8-10-r223] while the genomes of many other eukaryote groups are rich in introns (several per gene). [cite journal |author=Csurös M, Rogozin IB, Koonin EV |title=Extremely intron-rich genes in the alveolate ancestors inferred with a flexible maximum-likelihood approach |journal=Mol. Biol. Evol. |volume=25 |issue=5 |pages=903–11 |year=2008 |month=May |pmid=18296415 |doi=10.1093/molbev/msn039 |url=] [cite journal |author=Smith DR, Lee RW |title=Nucleotide diversity in the mitochondrial and nuclear compartments of Chlamydomonas reinhardtii: investigating the origins of genome architecture |journal=BMC Evol. Biol. |volume=8 |issue= |pages=156 |year=2008 |pmid=18495022 |pmc=2412866 |doi=10.1186/1471-2148-8-156]

Alternative splicing of introns within a gene may introduce greater variability of protein sequences translated from a single gene. The control of mRNA splicing is performed by a wide variety of signaling molecules.

Introns may also contain "old code", or sections of a gene that were once translated into a protein, but have since become inactive. It was generally assumed that the sequence of any given intron is junk DNA with no biological function. More recently, however, this is being disputed. [cite journal |author=Crosio C, Cecconi F, Mariottini P, Cesareni G, Brenner S, Amaldi F |title=Fugu intron oversize reveals the presence of U15 snoRNA coding sequences in some introns of the ribosomal protein S3 gene |journal=Genome Res. |volume=6 |issue=12 | pages = e15 |year=1996 |month=December |pmid=8973918 |url=http://www.genome.org/cgi/pmidlookup?view=long&pmid=8973918 | doi = 10.1101/gr.6.12.1227 ]

Introns contain several short sequences that are important for efficient splicing, such as acceptor and donor sites at either end of the intron as well as a branch point site, which are required for proper splicing by the spliceosome.

Discovery

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: [cite journal
author=Gilbert, Walter |authorlink=Walter Gilbert |year=1978 |title=Why genes in pieces |journal=Nature |volume=271 |issue=5645 |pages=501 |doi=10.1038/271501a0
]

"The notion of the cistron [...] must be replaced by that of a transcription unit containing regions which will be lost from the mature messenger - which I suggest we call introns (for intragenic regions) - alternating with regions which will be expressed - exons." (Gilbert 1978)

Classification of introns

Four classes of introns are known to exist:
*Nuclear introns
*Group I intron
*Group II intron
*Group III intron Some introns, such as the Group I and Group II introns, after transcription possess ribozyme activity, enabling them to catalyze their own splicing out of a primary RNA transcript. These introns are thus self splicing introns and are relatively rare compared to spliceosomal introns. This self-splicing activity 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.

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.

Group II and III introns are similar and have a conserved secondary structure. A so-called 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.Many self-splicing introns code for maturases that help with the splicing process, generally only the splicing of the intron that encodes it. [cite journal |author=Doetsch NA, Thompson MD, Hallick RB |title=A maturase-encoding group III twintron is conserved in deeply rooted euglenoid species: are group III introns the chicken or the egg? |journal=Mol. Biol. Evol. |volume=15 |issue=1 |pages=76–86 |year=1998 |month=January |pmid=9491607 |doi= |url=http://mbe.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=9491607]

Intron evolution

There are two competing theories that offer alternative scenarios for the origin and early evolution of spliceosomal introns. Other classes of introns such as self-splicing and tRNA introns are not subject to much debate, but see [ 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] for the former. These are popularly called as the Introns-Early (IE) or the Introns-Late (IL) views. [cite journal |author=Roy SW, Gilbert W |title=The evolution of spliceosomal introns: patterns, puzzles and progress |journal=Nat. Rev. Genet. |volume=7 |issue=3 |pages=211–21 |year=2006 |month=March |pmid=16485020 |doi=10.1038/nrg1807 |url=http://www.faculty.biol.ttu.edu/densmore/MB06pdfs/Nat.rev.intron%20evol.pdf]

The "IE model", championed by Walter Gilbert, [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.] 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. [Rodriguez-Trelles F, Tarrio R, Ayala, FJ. " [http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.genet.40.110405.090625?cookieSet=1 Origins and evolution of spliceosomal introns] ." "Annual Review of Genetics" 2006; 40: 47-76.] This model cannot account for some observed positional variation of introns shared among related genes. [Rzhetsky A, Ayala FJ. " [http://www.springerlink.com/content/lgvc0bly6j5wn3n0/fulltext.pdf The enigma of intron origins] ." "Cellular and Molecular Life Sciences" 1999; 55(1): 3-6.]

The "IL model" proposes that introns were more recently inserted into originally intron-less contiguous genes after the divergence of eukaryotes and prokaryotes. In this model, introns probably originated from transposable elements. This model is based on the observation that the spliceosomal introns are restricted to eukaryotes alone. However, there is considerable debate over the presence of introns in the early prokaryote-eukaryote ancestors and the subsequent intron loss-gain during eukaryotic evolution. [Sverdlov AV, Csuros M, Rogozin IB, Koonin EV. "A glimpse of a putative pre-intron phase of eukaryotic evolution." "Trends Genet." 2007 Mar; 23(3): 105-108. PMID 17239982] The evolution of introns and of the intron-exon structure may be largely independent of the evolution of coding-sequences. [cite journal |author=Yandell M, Mungall CJ, Smith C, "et al" |title=Large-scale trends in the evolution of gene structures within 11 animal genomes |journal=PLoS Comput. Biol. |volume=2 |issue=3 |pages=e15 |year=2006 |month=March |pmid=16518452 |doi=10.1371/journal.pcbi.0020015]

Identification

Nearly all eukaryotic nuclear introns begin with the nucleotide sequence GU, and end with AG (the GU-AG rule). These, along with a larger consensus sequence, help direct the splicing machinery to the proper intronic donor and acceptor sites.

ee also

Structure:
*Exon
*mRNA
*Eukaryotic gene example
*Eukaryotic chromosome fine structureSplicing:
*Alternate splicing
*Minor spliceosomeOthers:
*Intein
*Noncoding DNA
*Selfish DNA
*Twintron

References

External links

* [http://www.sgn.cornell.edu/tools/intron_detection/find_introns.pl Intron finding tool for plant genomic sequences]


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Look at other dictionaries:

  • intron — [ ɛ̃trɔ̃ ] n. m. • 1979; de intr(o) et on, par l angl. ♦ Biochim. Séquence d A. D. N. non codante, intervenant entre deux exons d un gène. Le gène de la globine comprend trois exons et deux introns. ● intron nom masculin Fraction d A.D.N.… …   Encyclopédie Universelle

  • intron — intron. См. интрон. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • intrón — (en genética molecular) secuencia de pares de bases del ADN que interrumpe la continuidad de la formación genética. Diccionario Mosby Medicina, Enfermería y Ciencias de la Salud, Ediciones Hancourt, S.A. 1999 …   Diccionario médico

  • Intron — Intron, nichtinformativer (nichtcodierender) Abschnitt in einem eukaryotischen Gen. I. werden transkribiert und nach der Transkription, beim Processing, durch einen Enzymkomplex aus der hnRNA herausgeschnitten. Die DNA Abschnitte, die… …   Deutsch wörterbuch der biologie

  • intron — 1978, from intragenic + ON (Cf. on) …   Etymology dictionary

  • intron — [in′trän΄] n. [coined (1978) by W. Gilbert (1932 ), U.S. molecular biologist < INTRA (because within the gene) + on, as in OPERON] an intervening sequence in the eukaryote genetic code, interrupting protein formation …   English World dictionary

  • Intron — schematischer Aufbau eines Gens Introns (englisch Intervening regions) sind die nicht codierenden Abschnitte der DNA innerhalb eines Gens (intragen), die benachbarte Exons trennen. Introns werden transkribiert, aber dann aus der prä mRNA …   Deutsch Wikipedia

  • Intron — Représentation schématique de l enchaînement exon intron dans un gène du génome, ici représenté par un chromosome. Un intron est une portion de gène non codante. Chez les organismes eucaryotes, les gènes qui codent les protéines sont constitués d …   Wikipédia en Français

  • Intrón — Un intrón es una región del ADN que debe ser eliminada del transcrito primario de ARN, a diferencia de los exones que son regiones que codifican para una determinada proteína. Los intrones son comunes en todos los tipos de ARN eucariota,… …   Wikipedia Español

  • Intrón — Un intrón es cada uno de los fragmentos de ADN en la codificación de un gen, que no son capaces de expresarse bajo la forma de una proteína. ● Es una secuencia no codificadora de ADN que separa a dos exones. El intrón inicialmente se transcribe… …   Enciclopedia Universal


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