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d loop wikipedia jump to content main menu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 in mitochondria 2 in telomeres 3 in dna repair toggle in dna repair subsection 3 1 meiotic recombination 4 see also 5 references toggle the table of contents d loop 10 languages العربية bosanski català deutsch 日本語 polski português srpskohrvatski српскохрватски српски srpski 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia dna structure this article is about dna structure for the loop of rna that forms the end of the d arm of a transfer rna molecule see d arm in molecular biology a displacement loop or d loop is a dna structure where the two strands of a double stranded dna molecule are separated for a stretch and held apart by a third strand of dna an r loop is similar to a d loop but in that case the third strand is rna rather than dna the third strand has a base sequence which is complementary to one of the main strands and pairs with it thus displacing the other complementary main strand in the region within that region the structure is thus a form of triple stranded dna a diagram in the paper introducing the term illustrated the d loop with a shape resembling a capital d where the displaced strand formed the loop of the d 1 d loops occur in a number of particular situations including in dna repair in telomeres and as a semi stable structure in mitochondrial circular dna molecules in mitochondria edit researchers at caltech discovered in 1971 that the circular mitochondrial dna from growing cells included a short segment of three strands which they called a displacement loop 1 they found the third strand was a replicated segment of the heavy strand or h strand of the molecule which it displaced and was hydrogen bonded to the light strand or l strand since then it has been shown that the third strand is the initial segment generated by a replication of the heavy strand that has been arrested shortly after initiation and is often maintained for some period in that state 2 the d loop occurs in the main non coding area of the mitochondrial dna molecule a segment called the control region or d loop region citation needed replication of the mitochondrial dna can occur in two different ways both starting in the d loop region 3 one way continues replication of the heavy strand through a substantial part e g two thirds of the circular molecule and then replication of the light strand begins the more recently reported mode starts at a different origin within the d loop region and uses coupled strand replication with simultaneous synthesis of both strands 3 4 certain bases within the d loop region are conserved but large parts are highly variable and the region has proven to be useful for the study of the evolutionary history of vertebrates 5 the region contains promoters for the transcription of rna from the two strands of mitochondrial dna immediately adjacent to the d loop structure that is associated with initiation of dna replication 6 d loop sequences are also of interest in the study of cancers 7 the function of the d loop is not yet clear but recent research suggests that it participates in the organization of the mitochondrial nucleoid 8 9 the single third strand is also called 7s dna the primer used for 7s dna synthesis is called 7s rna 10 in telomeres edit in 1999 it was reported that telomeres which cap the end of chromosomes terminate in a lariat like structure termed a t loop telomere loop 11 this is a loop of both strands of the chromosome which are joined to an earlier point in the double stranded dna by the 3 strand end invading the strand pair to form a d loop the joint is stabilized by the shelterin protein pot1 12 the t loop which is completed by the d loop splice protects the end of the chromosome from damage 13 in dna repair edit when a double stranded dna molecule has suffered a break in both strands one repair mechanism available in diploid eukaryotic cells is homologous recombination repair this makes use of the intact chromosome homologous to the broken one as a template to bring the two double stranded pieces into correct alignment for rejoining early in this process one strand of one piece is matched to a strand of the intact chromosome and that strand is used to form a d loop at that point displacing the intact chromosome s other strand various ligation and synthesis steps follow to effect the rejoining 14 in humans the protein rad51 is central to the homologous search and formation of the d loop in the bacterium escherichia coli a similar function is performed by the protein reca 15 meiotic recombination edit a current model of meiotic recombination initiated by a double strand break or gap followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process repair of the gap can lead to crossover co or non crossover nco of the flanking regions co recombination is thought to occur by the double holliday junction dhj model illustrated on the right above nco recombinants are thought to occur primarily by the synthesis dependent strand annealing sdsa model illustrated on the left above most recombination events appear to be the sdsa type during meiosis repair of double strand damages particularly double strand breaks occurs by the recombination process outlined in the accompanying diagram as shown in the diagram a d loop plays a central role in meiotic recombinational repair of such damages during this process rad51 and dmc1 recombinases bind the 3 single strand dna ssdna tails to form helical nucleoprotein filaments that perform a search for intact homologous double stranded dna dsdna 16 once the homologous sequence is found the recombinases facilitate invasion of the ssdna end into the homologous dsdna to form a d loop after strand exchange homologous recombination intermediates are processed by either of two distinct pathways see diagram to form the final recombinant chromosomes citation needed see also edit d loop replication mtdna control region references edit 1 2 kasamatsu h robberson d l vinograd j 1971 a novel closed circular mitochondrial dna with properties of a replicating intermediate proceedings of the national academy of sciences of the united states of america 68 9 2252 2257 bibcode 1971pnas 68 2252k doi 10 1073 pnas 68 9 2252 pmc 389395 pmid 5289384 doda j n wright c t clayton d a 1981 elongation of displacement loop strands in human and mouse mitochondrial dna is arrested near specific template sequences proceedings of the national academy of sciences of the united states of america 78 10 6116 6120 bibcode 1981pnas 78 6116d doi 10 1073 pnas 78 10 6116 pmc 348988 pmid 6273850 1 2 fish j raule n attardi g 2004 discovery of a major d loop replication origin reveals two modes of human mtdna synthesis pdf science 306 5704 2098 2101 bibcode 2004sci 306 2098f doi 10 1126 science 1102077 pmid 15604407 s2cid 36033690 holt i j lorimer h e jacobs h t 2000 coupled leading and lagging strand synthesis of mammalian mitochondrial dna cell 100 5 515 524 doi 10 1016 s0092 8674 00 80688 1 pmid 10721989 larizza a pesole g reyes a sbisà e saccone c 2002 lineage specificity of the evolutionary dynamics of the mtdna d loop region in rodents journal of molecular evolution 54 2 145 155 bibcode 2002jmole 54 145l doi 10 1007 s00239 001 0063 4 pmid 11821908 s2cid 40529707 chang d d clayton d a 1985 priming of human mitochondrial dna replication occurs at the light strand promoter proceedings of the national academy of sciences of the united states of america 82 2 351 355 bibcode 1985pnas 82 351c doi 10 1073 pnas 82 2 351 pmc 397036 pmid 2982153 akouchekian m houshmand m hemati s ansaripour m shafa m 2009 high rate of mutation in mitochondrial dna displacement loop region in human colorectal cancer diseases of the colon rectum 52 3 526 530 doi 10 1007 dcr 0b013e31819acb99 pmid 19333057 s2cid 28775491 he j mao c c reyes a sembongi h di re m granycome c clippingdale a b fearnley i m harbour m robinson a j reichelt s spelbrink j n walker j e holt i j 2007 the aaa protein atad3 has displacement loop binding properties and is involved in mitochondrial nucleoid organization the journal of cell biology 176 2 141 146 doi 10 1083 jcb 200609158 pmc 2063933 pmid 17210950 leslie m 2007 thrown for a d loop the journal of cell biology 176 2 129a doi 10 1083 jcb 1762iti3 pmc 2063944 nicholls thomas j minczuk michal august 2014 in d loop 40years of mitochondrial 7s dna experimental gerontology 56 175 181 doi 10 1016 j exger 2014 03 027 griffith j d comeau l rosenfield s stansel r m bianchi a moss h de lange t 1999 mammalian telomeres end in a large duplex loop cell 97 4 503 514 doi 10 1016 s0092 8674 00 80760 6 pmid 10338214 maestroni l matmati s coulon s 2017 solving the telomere replication problem genes 8 2 e55 doi 10 3390 genes8020055 pmc 5333044 pmid 28146113 greider c w 1999 telomeres do d loop t loop cell 97 4 419 422 doi 10 1016 s0092 8674 00 80750 3 pmid 10338204 hartl daniel l jones elizabeth w 2005 page 251 genetics analysis of genes and genomes jones bartlett publishers isbn 978 0763715113 shibata t nishinaka t mikawa t aihara h kurumizaka h yokoyama s ito y 2001 homologous genetic recombination as an intrinsic dynamic property of a dna structure induced by reca rad51 family proteins a possible advantage of dna over rna as genomic material proceedings of the national academy of sciences of the united states of america 98 15 8425 8432 bibcode 2001pnas 98 8425s doi 10 1073 pnas 111005198 pmc 37453 pmid 11459985 sansam cl pezza rj 2015 connecting by breaking and repairing mechanisms of dna strand exchange in meiotic recombination febs j 282 13 2444 57 doi 10 1111 febs 13317 pmc 4573575 pmid 25953379 retrieved from https en wikipedia org w index php title d loop oldid 1373466201 categories dna telomeres nucleic acid secondary structures hidden categories articles with short description short description matches wikidata all articles with unsourced statements articles with unsourced statements from november 2024 this page was last edited on 6 september 2026 at 03 26 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by using this site you agree to the terms of use and privacy policy wikipedia is a registered trademark 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