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RNA世界學說：修订间差异

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{{Main|核糖开关}}
{{Main|核糖开关}}
核糖开关作为基因表达的调控物质之一已在细菌、植物和[[古菌]]中发现。核糖开关会改变其[[二级结构]]以响应所结合的{{le|代谢物|metabolite}}。这一结构改变会形成或截断[[終止子]]，从而允许或中断转录进行<ref>{{cite journal | author = Nudler E, Mironov AS | title = The riboswitch control of bacterial metabolism | journal = Trends Biochem Sci | volume =29| issue = 1 | pages = 11–7 | year = 2004 | pmid = 14729327 | doi = 10.1016/j.tibs.2003.11.004 }}</ref>。另外，核糖开关还可以结合或阻隔[[夏因-达尔加诺序列|SD序列]]来影响转录<ref>{{cite journal | author = Tucker BJ, Breaker RR | title = Riboswitches as versatile gene control elements | journal = Current Opinion in Structural Biology | volume = 15| issue = 3 | pages = 342–8 | year = 2005 | pmid = 15919195| doi = 10.1016/j.sbi.2005.05.003 }}</ref>。这些核糖开关可能源自RNA世界<ref>Switching the light on plant riboswitches. Samuel Bocobza and Asaph Aharoni Trends in Plant Science Volume 13, Issue 10, October 2008, Pages 526-533 {{doi|10.1016/j.tplants.2008.07.004}} PMID 18778966</ref>。此外，{{le|RNA温度计|RNA thermometer}}也能受温度变化而变构调节基因表达<ref name="Nar06">{{cite journal |author=Narberhaus F, Waldminghaus T, Chowdhury S |title=RNA thermometers |journal=FEMS Microbiol. Rev. |volume=30 |issue=1 |pages=3–16 |date=January 2006 |pmid=16438677 |doi=10.1111/j.1574-6976.2005.004.x |url=http://onlinelibrary.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=0168-6445&date=2006&volume=30&issue=1&spage=3 |accessdate=2011-04-23}}</ref>。
核糖开关作为基因表达的调控物质之一已在细菌、植物和[[古菌]]中发现。核糖开关会改变其[[二级结构]]以响应所结合的{{le|代谢物|metabolite}}。这一结构改变会形成或截断[[終止子]]，从而允许或中断转录进行<ref>{{cite journal | author = Nudler E, Mironov AS | title = The riboswitch control of bacterial metabolism | journal = Trends Biochem Sci | volume =29| issue = 1 | pages = 11–7 | year = 2004 | pmid = 14729327 | doi = 10.1016/j.tibs.2003.11.004 }}</ref>。另外，核糖开关还可以结合或阻隔[[夏因-达尔加诺序列|SD序列]]来影响转录<ref>{{cite journal | author = Tucker BJ, Breaker RR | title = Riboswitches as versatile gene control elements | journal = Current Opinion in Structural Biology | volume = 15| issue = 3 | pages = 342–8 | year = 2005 | pmid = 15919195| doi = 10.1016/j.sbi.2005.05.003 }}</ref>。这些核糖开关可能源自RNA世界<ref>Switching the light on plant riboswitches. Samuel Bocobza and Asaph Aharoni Trends in Plant Science Volume 13, Issue 10, October 2008, Pages 526-533 {{doi|10.1016/j.tplants.2008.07.004}} PMID 18778966</ref>。此外，{{le|RNA温度计|RNA thermometer}}也能受温度变化而变构调节基因表达<ref name="Nar06">{{cite journal |author=Narberhaus F, Waldminghaus T, Chowdhury S |title=RNA thermometers |journal=FEMS Microbiol. Rev. |volume=30 |issue=1 |pages=3–16 |date=January 2006 |pmid=16438677 |doi=10.1111/j.1574-6976.2005.004.x |url=http://onlinelibrary.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=0168-6445&date=2006&volume=30&issue=1&spage=3 |accessdate=2011-04-23}}</ref>。
==当前难点==
RNA世界假说能被诸如RNA能像[[DNA]]一样存储、传递、复制[[遗传学|遗传]]信息；RNA能作为[[核酶]]进行催化等证据支持，因它能执行DNA和蛋白质的任务，故被认为是生命起源的物质形式<ref name="Atk06" />。一些[[病毒]]也使用RNA而不是DNA作为其遗传信息载体<ref>Patton, John T. Editor (2008). Segmented Double-stranded RNA Viruses: Structure and Molecular Biology. Caister Academic Press. Editor's affiliation: Laboratory of Infectious Diseases, NIAID, NIH, Bethesda, MD 20892-8026. ISBN 978-1-904455-21-9</ref>。虽然[[核苷酸]]并未在[[米勒-尤里实验|米勒-尤里]]关于[[生命起源]]的实验中出现，但它们可能的[[前体]]已有报道<ref name=Powner />，[[嘌呤]]碱基如[[腺嘌呤]]可能由[[氰化氢]]{{le|五聚体|pentamer|五聚化}}生成。对{{le|Qβ噬菌体|Bacteriophage Qβ}}RNA的实验也展示了RNA的自我复制能力<!-- I don't want to link to Spiegelman Monster here because I'm not sure that this is what's being talked about, but it appears to be. --><ref>Bell, Graham: The Basics of Selection. Springer, 1997.</ref>
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Since there were no known chemical pathways for the abiogenic synthesis of nucleotides from [[pyrimidine]] nucleobases [[cytosine]] and [[uracil]] under prebiotic conditions, it is thought by some that nucleic acids did not contain these [[nucleobase]]s seen in life's nucleic acids.<ref>{{cite journal |last=Orgel |first=L. |authorlink= |year=1994 |title=The origin of life on earth |journal=Scientific American |volume=271 |issue=4 |pages=81 |doi=10.1038/scientificamerican1094-76 |pmid=7524147}}</ref> The nucleoside cytosine has a half-life in isolation of 19 days at {{convert|100|°C|°F|abbr=on}} and 17,000 years in freezing water, which some argue is too short on the [[geologic time scale]] for accumulation.<ref>{{cite journal |last=Levy |first=Matthew |authorlink= |author2=Miller, Stanley L. |year=1998 |title=The stability of the RNA bases: Implications for the origin of life |journal=[[Proceedings of the National Academy of Sciences|PNAS]] |volume=95 |issue=14 |pages=7933–7938 |doi=10.1073/pnas.95.14.7933|pmid=9653118 |pmc=20907|bibcode = 1998PNAS...95.7933L }}</ref> Others have questioned whether [[ribose]] and other backbone sugars could be stable enough to find in the original genetic material,<ref>{{cite journal |last=Larralde |first=R. |authorlink= |author2=Robertson, M. P. |author3=Miller, S. L. |year=1995 |title=Rates of decomposition of ribose and other sugars: implications for chemical evolution |journal=PNAS |volume=92 |issue=18 |pages=8158–8160|doi=10.1073/pnas.92.18.8158 |pmid=7667262 |pmc=41115 |bibcode = 1995PNAS...92.8158L }}</ref> and have raised the issue that all ribose molecules would have had to be the same [[enantiomer]], as any nucleotide of the wrong [[chirality (chemistry)|chirality]] acts as a chain [[terminator (genetics)|terminator]].<ref>{{cite journal |title=Chiral selection in poly(C)-directed synthesis of oligo(G) |author=Joyce GF |author2=et al. |journal=Nature |pmid=6462250 |year=1984 |issue=5978 |volume=310 |pages=602–604 |doi=10.1038/310602a0|bibcode = 1984Natur.310..602J }}</ref>

Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesised by a sequence of reactions that by-pass free sugars and assemble in a stepwise fashion by going against the dogma that nitrogenous and oxygenous chemistries should be avoided. In a series of publications, The ''[[John Sutherland (chemist)|Sutherland]] Group'' at the School of Chemistry, [[University of Manchester]] have demonstrated high yielding routes to [[cytidine]] and [[uridine]] ribonucleotides built from small 2 and 3 carbon fragments such as [[glycolaldehyde]], [[glyceraldehyde]] or glyceraldehyde-3-phosphate, [[cyanamide]] and [[cyanoacetylene]]. One of the steps in this sequence allows the isolation of [[enantiomer|enantiopure]] ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater, of possible interest towards biological homochirality.<ref>Direct Assembly of Nucleoside Precursors from Two- and Three-Carbon Units Carole Anastasi, Michael A. Crowe, Matthew W. Powner, John D. Sutherland Angewandte Chemie International Edition Volume 45, Issue 37 , Pages 6176–79</ref> This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallised out from a mixture of the other pentose [[aminooxazoline]]s. Aminooxazolines can react with cyanoacetylene in a mild and highly efficient manner, controlled by inorganic phosphate, to give the cytidine ribonucleotides. Photoanomerization with [[UV light]] allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry, one problem with this chemistry is the selective phosphorylation of alpha-cytidine at the 2' position.<ref>Potentially Prebiotic Synthesis of Pyrimidine β-D-Ribonucleotides by Photoanomerization/Hydrolysis of α-D-Cytidine-2′-Phosphate Matthew W. Powner, John D. Sutherland ChemBioChem Volume 9, Issue 15 , Pages 2386–87</ref> However, in 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerise into RNA.<ref name=Powner2009/> This was hailed as strong evidence for the RNA world.<ref>{{cite journal | title=RNA world easier to make| author=Van Noorden R| journal=Nature| year=2009| url=http://www.nature.com/news/2009/090513/full/news.2009.471.html| doi= 10.1038/news.2009.471 }}</ref> The paper also highlighted the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates.<ref name=Powner2009>{{cite journal | title=Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions| author=Powner MW, Gerland B, Sutherland JD| journal=Nature| year=2009| volume=459| pages=239–242| doi=10.1038/nature08013 | pmid=19444213 | issue=7244|bibcode = 2009Natur.459..239P }}</ref> A potential weakness of these routes is the generation of enantioenriched glyceraldehyde, or its 3-phosphate derivative (glyceraldehyde prefers to exist as its keto [[tautomer]] dihydroxyacetone).{{Citation needed|date=May 2009}}

On August 8, 2011, a report, based on [[NASA]] studies with [[meteorites]] found on [[Earth]], was published suggesting building blocks of [[RNA]] ([[adenine]], [[guanine]] and related [[organic molecules]]) may have been formed extraterrestrially in [[outer space]].<ref name="Callahan">{{cite journal |last1=Callahan |first1=M.P. |last2=Smith |first2=K.E. |last3=Cleaves |first3=H.J. |last4=Ruzicka |first4=J. |last5=Stern |first5=J.C. |last6=Glavin |first6=D.P. |last7=House |first7=C.H. |last8=Dworkin |first8=J.P. |date=11 August 2011 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |url=http://www.pnas.org/content/early/2011/08/10/1106493108 |publisher=[[PNAS]] |doi=10.1073/pnas.1106493108 |accessdate=2011-08-15 |bibcode = 2011PNAS..10813995C |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=34 |pages=13995 }}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space|url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html|publisher=[[NASA]] |date=8 August 2011 |accessdate=2011-08-10}}</ref><ref name="DNA">{{cite web |author=ScienceDaily Staff |title=DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests|url=http://www.sciencedaily.com/releases/2011/08/110808220659.htm |date=9 August 2011 |publisher=[[ScienceDaily]] |accessdate=2011-08-09}}</ref> On August 29, 2012, and in a world first, astronomers at [[Copenhagen University]] reported the detection of a specific sugar molecule, [[glycolaldehyde]], in a distant star system. The molecule was found around the [[protostar|protostellar]] binary ''IRAS 16293-2422'', which is located 400 light years from Earth.<ref name="NG-20120829">{{cite journal|title=Sugar Found In Space|journal=National Geographic |last=Than |first=Ker |date=August 29, 2012 |url=http://news.nationalgeographic.com/news/2012/08/120829-sugar-space-planets-science-life/ |accessdate=August 31, 2012 }}</ref><ref name="AP-20120829">{{cite web |author=Staff |title=Sweet! Astronomers spot sugar molecule near star |url=http://apnews.excite.com/article/20120829/DA0V31D80.html |date=August 29, 2012 |publisher=[[AP News]] |accessdate=August 31, 2012 }}</ref> Glycolaldehyde is needed to form [[ribonucleic acid]], or [[RNA]], which is similar in function to [[DNA]]. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.<ref>{{cite journal|title=Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA|author=Jørgensen, J. K.|author2=Favre, C. |author3=Bisschop, S. |author4=Bourke, T. |author5=Dishoeck, E. |author6= Schmalzl, M. |version=eprint |year=2012|url=http://www.eso.org/public/archives/releases/sciencepapers/eso1234/eso1234a.pdf|bibcode=2012ApJ...757L...4J|volume=757|pages=L4|journal=The Astrophysical Journal Letters|doi=10.1088/2041-8205/757/1/L4}}</ref>

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===“分子生物学之梦”===
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"Molecular biologist's dream" is a phrase coined by [[Gerald Joyce]] and [[Leslie Orgel]] to refer to the problem of emergence of [[self-replicating]] [[RNA]] molecules, as any movement towards an RNA world on a properly modeled prebiotic [[early Earth]] would have been continuously suppressed by destructive reactions.<ref name="arn">{{cite web|url=http://www.arn.org/docs/odesign/od171/rnaworld171.htm |title=The RNA World: A Critique|publisher =[[Access Research Network]]|author=Gordon C. Mills, Dean Kenyon|accessdate =10 Sep 2011}}</ref> It was noted that many of the steps needed for the [[nucleotides]] formation do not proceed efficiently in [[Abiogenesis#Early conditions|prebiotic]] conditions.<ref>{{cite book | last = Schopf| first = J. William| title = Life's origin: the beginnings of biological evolution| publisher =University of California Press|year =2002| page = 150| isbn =0-520-23390-5}}</ref> Joyce and Orgel specifically referred the molecular biologist's dream to "a magic [[catalyst]]" that could "convert the activated nucleotides to a random ensemble of [[polynucleotide]] sequences, a subset of which had the ability to replicate".<ref name="arn"/>

Joyce and Orgel further argued that nucleotides cannot link unless there is some [[Activation#Biochemistry|activation]] of the [[phosphate group]], whereas the only effective activating groups for this are "totally implausible in any prebiotic scenario", particularly [[adenosine triphosphate]].<ref name="arn"/> According to Joyce and Orgel, in case of the phosphate group activation, the basic [[polymer]] product would have [[5',5'-pyrophosphate]] linkages, while the [[3',5'-phosphodiester]] linkages, which are present in all known RNA, would be much less abundant.<ref name="arn"/> The associated molecules would have been also prone to addition of incorrect nucleotides or to reactions with numerous other substances likely to have been present.<ref name="arn"/> The RNA molecules would have been also continuously degraded by such destructive process as spontaneous [[hydrolysis]], present on the early Earth.<ref name="arn"/> Joyce and Orgel proposed to reject "the myth of a self-replicating RNA molecule that arose ''de novo'' from a soup of random polynucleotides"<ref name="arn"/> and hypothesised about a scenario where the prebiotic processes furnish pools of [[enantiopure]] [[beta-D-ribonucleosides]].<ref>{{cite web|url=http://gow.epsrc.ac.uk/ViewGrant.aspx?GrantRef=EP/E032753/1|title=Prebiotic RNA chemistry: realising the molecular biologist's dream|publisher =Engineering and Physical Sciences Research Council|accessdate =10 Sep 2011}}</ref>
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== 參考文獻 ==
== 參考文獻 ==

2014年9月29日 (一) 05:18的版本

對比RNA（左）與DNA（右），顯示了螺旋和每個採用的核鹼基。
首先提出RNA世界一詞的生物學家沃特·吉爾伯特。

RNA世界學說（英語：RNA world hypothesis）是一個理論，認為地球上早期的生命分子以RNA先出現，之後才是DNA。且這些早期的RNA分子同時擁有如同DNA的遺傳訊息儲存功能，以及如蛋白質般的催化能力，支持了早期的細胞或前細胞生命的運作。

關於獨立的RNA生命型態的概念，是在1968年由卡爾·沃斯（Carl Woese）所著的《遺傳密碼》（The Genetic Code）一書中所建立[1]，虽然当时该理论还不叫那个名字。此外亞歷山大·里奇（英语：Alexander Rich）也曾於1963年提出類似想法。「RNA世界」一詞則是由諾貝爾獎得主沃特·吉爾伯特（Walter Gilbert）於1986年提出，是依據現今RNA具有各種不同型態的催化性質所做的推論[2]。

历史

在研究生命起源过程中的一大问题就是所有现存生物所使用的信息复制系统和能量代谢体系都涉及三种不同类型的生物大分子（DNA、RNA和蛋白质）之间的紧密合作。这似乎表明生命不可能由较简单的形式逐步进化，而是一步到位变成当前这个缺一不可的体系。而最早提出RNA可能是三者中最原始分子[3]的是弗朗西斯·克里克[4]和莱斯利·奥格尔[5]以及卡尔·乌斯（在其1967年的书The Genetic Code《遗传密码》[6]）。另外，麻省理工学院的分子生物学家亚历山大·里奇在1962年的一篇纪念诺奖得主圣捷尔吉·阿尔伯特的文章中也有类似想法[7]。汉斯·库恩（英语：Hans Kuhn）在1972年提出了现代的基因系统可能源于一个基于核苷酸的前体。这促使了哈罗德·怀特在1976年观察到许多酶的必需辅因子是核苷酸或核苷酸衍生物，他提出这些核苷酸辅因子代表了“核酸酶的化石”（"fossils of nucleic acid enzymes"）[8]。而“RNA世界”一词（"RNA World"）则是由诺奖得主沃特·吉爾伯特在1986年提出，来表示具有催化性质的可自我复制的RNA是最早的生物大分子的假说[9]。

RNA的属性

RNA的一些性质使RNA世界假说在理论上是可行的，但作为生命的起源仍需更进一步的证据[7]。已知RNA能进行有效的催化作用，并且它与DNA的相似性也显明它能作为生物信息的存储物质。但对于RNA是否是第一个自发的自我复制系统（“RNA第一”假说），还是RNA是之前可能存在的别的系统的进化产物任然众说纷纭[3]。有一个观点认为不同类型的核酸，被称为前RNA（pre-RNA）是第一个能进行自我复制的分子，之后被RNA所取代。另外一些观点认为，最近发现的一些有活性的核酸类似物，如肽核酸（PNA）、蘇糖核酸（TNA）、甘油核酸（GNA）等[10][11]也具有作为生命起源物质的可能性[12]，故现在确定“RNA第一”还为时尚早[3]。虽然和RNA比这些核酸类似物在结构上较为“简单”，但在化学上难以说清RNA是从这些“较简单”的物质进化而来[13]。

RNA作为酶

更多信息：核酶

具有催化作用的RNA称为核酶，在生命基于DNA的今天被称为分子活化石。核酶在一些生物过程（英语：Biological process）中起重要作用，比如核糖体，是蛋白质合成的关键。其它核酶也有许多不同功能，锤头状核酶（英语：hammerhead ribozyme）能自我切割[14]，RNA聚合酶的一个核酶能自我催化自身的合成[15]。

在生命起源中酶所需的重要性质有：

  • 具有自我复制的能力，或复制其它的RNA分子。在实验室中，一些较短的RNA已证明可以复制其它RNA。其中最短的为165-碱基长，但据估计只有其中的一部分参与了复制功能。
  • 催化简单化学反应的能力——即RNA分子能通过折叠形成催化中心。在实验室中，一些相对较短的RNA分子已具有该能力[16][17]。
  • 在RNA的3'-端结合氨基酸的能力，以使用其侧链基团的化学性质[18]。
  • 催化肽键形成的能力，以生成短肽乃至更长的蛋白质。这一任务在现代的细胞中由核糖体完成。核糖体是由几个RNA（称为rRNA）和一些蛋白质（称为核糖体蛋白质）组成的复合体，其中rRNA负责催化，核糖体蛋白质上的氨基酸残基都距离活性位点的18Å以上[7]。在实验室中合成了更短的能催化肽键生成的RNA，这暗示着rRNA可能由更短的RNA进化而来[19]。它也表明，氨基酸在进化出复杂的肽链之前，是以辅因子的形式参与RNA的反应，以提高其活性或使反应更多样化。类似地，tRNA在作为转运氨基酸的载体之前可能另有他用[20]。

RNA作为信息存储介质

RNA与DNA分子非常相似，在化学上只有两点不同，这使得生物信息在RNA上的存储方式与DNA类似，而由于RNA通常只有单链，DNA形成了双链螺旋，故DNA作为存储介质更为稳定。

RNA和DNA的主要不同在于糖的2'-位多了个羟基基团

DNA和RNA的结构比较

主条目：RNA和DNA

RNA和DNA的主要不同在于RNA的核糖比DNA多了个羟基（见右侧图）[7]。但这个基团会使RNA更加不稳定，2'位的羟基基团会攻击3'位的羟基的磷酸二酯鍵，从而使磷酸二酯骨架裂解。2'位羟基的存在还使RNA在构象上不能形成像DNA那样的B型双螺旋，而只能形成较不稳定的A型双螺旋（无论是RNA-RNA双链还是RNA-DNA双链都只能是A型的双螺旋）。

已隱藏部分未翻譯内容，歡迎參與翻譯。

RNA also uses a different set of bases than DNA—adenine, guanine, cytosine and uracil, instead of adenine, guanine, cytosine and thymine. Chemically, uracil is similar to thymine, differing only by a methyl group, and its production requires less energy.[21] In terms of base pairing, this has no effect. Adenine readily binds uracil or thymine. Uracil is, however, one product of damage to cytosine that makes RNA particularly susceptible to mutations that can replace a GC base pair with a GU (wobble) or AU base pair.

RNA is thought to have preceded DNA, because of their ordering in the biosynthetic pathways. The deoxyribonucleotides used to make DNA are made from ribonucleotides, the building blocks of RNA, by removing the 2'-hydroxyl group. As a consequence a cell must have the ability to make RNA before it can make DNA.

RNA信息存储的局限性

RNA的化学性质使得大RNA分子本身比较脆弱。他们可以很容易地水解成构成自身的核苷酸。[22][23] 这些局限并没有使RNA不能储存信息，不过由于一些能量需要用来修补和替换损坏的RNA分子，这种储存方式会更加耗费能量。而且变异的可能性也会增加。虽然这些特性使得RNA不适合用于今天的“DNA优化”的生命体，但是对于更加原始的生命体来说，这些也许是可以接受的。

RNA作为调控物质

主条目：核糖开关

核糖开关作为基因表达的调控物质之一已在细菌、植物和古菌中发现。核糖开关会改变其二级结构以响应所结合的代谢物。这一结构改变会形成或截断終止子，从而允许或中断转录进行[24]。另外，核糖开关还可以结合或阻隔SD序列来影响转录[25]。这些核糖开关可能源自RNA世界[26]。此外，RNA温度计（英语：RNA thermometer）也能受温度变化而变构调节基因表达[27]。

当前难点

RNA世界假说能被诸如RNA能像DNA一样存储、传递、复制遗传信息；RNA能作为核酶进行催化等证据支持，因它能执行DNA和蛋白质的任务，故被认为是生命起源的物质形式[7]。一些病毒也使用RNA而不是DNA作为其遗传信息载体[28]。虽然核苷酸并未在米勒-尤里关于生命起源的实验中出现，但它们可能的前体已有报道[12]，嘌呤碱基如腺嘌呤可能由氰化氢五聚化（英语：pentamer）生成。对Qβ噬菌体（英语：Bacteriophage Qβ）RNA的实验也展示了RNA的自我复制能力[29]

已隱藏部分未翻譯内容，歡迎參與翻譯。

Since there were no known chemical pathways for the abiogenic synthesis of nucleotides from pyrimidine nucleobases cytosine and uracil under prebiotic conditions, it is thought by some that nucleic acids did not contain these nucleobases seen in life's nucleic acids.[30] The nucleoside cytosine has a half-life in isolation of 19 days at 100 °C（212 °F） and 17,000 years in freezing water, which some argue is too short on the geologic time scale for accumulation.[31] Others have questioned whether ribose and other backbone sugars could be stable enough to find in the original genetic material,[32] and have raised the issue that all ribose molecules would have had to be the same enantiomer, as any nucleotide of the wrong chirality acts as a chain terminator.[33]

Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesised by a sequence of reactions that by-pass free sugars and assemble in a stepwise fashion by going against the dogma that nitrogenous and oxygenous chemistries should be avoided. In a series of publications, The Sutherland Group at the School of Chemistry, University of Manchester have demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene. One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater, of possible interest towards biological homochirality.[34] This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallised out from a mixture of the other pentose aminooxazolines. Aminooxazolines can react with cyanoacetylene in a mild and highly efficient manner, controlled by inorganic phosphate, to give the cytidine ribonucleotides. Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry, one problem with this chemistry is the selective phosphorylation of alpha-cytidine at the 2' position.[35] However, in 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerise into RNA.[36] This was hailed as strong evidence for the RNA world.[37] The paper also highlighted the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates.[36] A potential weakness of these routes is the generation of enantioenriched glyceraldehyde, or its 3-phosphate derivative (glyceraldehyde prefers to exist as its keto tautomer dihydroxyacetone).[來源請求]

On August 8, 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting building blocks of RNA (adenine, guanine and related organic molecules) may have been formed extraterrestrially in outer space.[38][39][40] On August 29, 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth.[41][42] Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.[43]

“分子生物学之梦”

已隱藏部分未翻譯内容，歡迎參與翻譯。

"Molecular biologist's dream" is a phrase coined by Gerald Joyce and Leslie Orgel to refer to the problem of emergence of self-replicating RNA molecules, as any movement towards an RNA world on a properly modeled prebiotic early Earth would have been continuously suppressed by destructive reactions.[44] It was noted that many of the steps needed for the nucleotides formation do not proceed efficiently in prebiotic conditions.[45] Joyce and Orgel specifically referred the molecular biologist's dream to "a magic catalyst" that could "convert the activated nucleotides to a random ensemble of polynucleotide sequences, a subset of which had the ability to replicate".[44]

Joyce and Orgel further argued that nucleotides cannot link unless there is some activation of the phosphate group, whereas the only effective activating groups for this are "totally implausible in any prebiotic scenario", particularly adenosine triphosphate.[44] According to Joyce and Orgel, in case of the phosphate group activation, the basic polymer product would have 5',5'-pyrophosphate linkages, while the 3',5'-phosphodiester linkages, which are present in all known RNA, would be much less abundant.[44] The associated molecules would have been also prone to addition of incorrect nucleotides or to reactions with numerous other substances likely to have been present.[44] The RNA molecules would have been also continuously degraded by such destructive process as spontaneous hydrolysis, present on the early Earth.[44] Joyce and Orgel proposed to reject "the myth of a self-replicating RNA molecule that arose de novo from a soup of random polynucleotides"[44] and hypothesised about a scenario where the prebiotic processes furnish pools of enantiopure beta-D-ribonucleosides.[46]

參考文獻

  1. ^ Woese, Carl. The Genetic Code. Harper & Row. 1968. ISBN 978-0060471767.  已忽略未知参数|month=（建议使用|date=） (帮助)
  2. ^ Gilbert, Walter. The RNA World. Nature. 1986, 319: 618. doi:10.1038/319618a0.  已忽略未知参数|month=（建议使用|date=） (帮助)
  3. ^ 3.0 3.1 3.2 引用错误：没有为名为Cech的参考文献提供内容
  4. ^ Crick FH. The origin of the genetic code. J Mol Biol. 1968, 38 (3): 367–379. PMID 4887876. doi:10.1016/0022-2836(68)90392-6. 
  5. ^ Orgel LE. Evolution of the genetic apparatus. J Mol Biol. 1968, 38 (3): 381–393. PMID 5718557. doi:10.1016/0022-2836(68)90393-8. 
  6. ^ Woese C.R. (1967). The genetic code: The molecular basis for genetic expression. p. 186. Harper & Row
  7. ^ 7.0 7.1 7.2 7.3 7.4 Atkins, John F.; Gesteland, Raymond F.; Cech, Thomas. The RNA world: the nature of modern RNA suggests a prebiotic RNA world. Plainview, N.Y: Cold Spring Harbor Laboratory Press. 2006. ISBN 0-87969-739-3. 
  8. ^ White, HB III. Coenzymes as Fossils of an Earlier Metabolic State. J Mol Evol. 1976, 7 (2): 101–104. PMID 1263263. doi:10.1007/BF01732468. 
  9. ^ Gilbert, Walter. The RNA World. Nature. February 1986, 319 (6055): 618. Bibcode:1986Natur.319..618G. doi:10.1038/319618a0. 
  10. ^ Orgel, Leslie. A Simpler Nucleic Acid. Science. November 2000, 290 (5495): 1306–7. PMID 11185405. doi:10.1126/science.290.5495.1306. 
  11. ^ Nelson, K.E.; Levy, M.; Miller, S.L. Peptide nucleic acids rather than RNA may have been the first genetic molecule. Proc. Natl. Acad. Sci. USA. April 2000, 97 (8): 3868–71. Bibcode:2000PNAS...97.3868N. PMC 18108 可免费查阅. PMID 10760258. doi:10.1073/pnas.97.8.3868. 
  12. ^ 12.0 12.1 Powner M.W., Gerland B, Sutherland J.D. Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions. Nature. 2009, 459 (7244): 239–242. Bibcode:2009Natur.459..239P. PMID 19444213. doi:10.1038/nature08013. 
  13. ^ Sutherland, J.D; Anastasi, C., Buchet F.F, Crower M.A, Parkes A.L, Powner M. W., Smith J.M. RNA: Prebiotic Product, or Biotic Invention. Chemistry & Biodiversity. April 2007, 4 (4): 721–739. PMID 17443885. doi:10.1002/cbdv.200790060. 
  14. ^ Forster AC, Symons RH. Self-cleavage of plus and minus RNAs of a virusoid and a structural model for the active sites. Cell. 1987, 49 (2): 211–220. PMID 2436805. doi:10.1016/0092-8674(87)90562-9. 
  15. ^ Johnston W, Unrau P, Lawrence M, Glasner M, Bartel D. RNA-catalyzed RNA polymerization: accurate and general RNA-templated primer extension (PDF). Science. 2001, 292 (5520): 1319–25. Bibcode:2001Sci...292.1319J. PMID 11358999. doi:10.1126/science.1060786. 
  16. ^ Huang, Yang, and Yarus, RNA enzymes with two small-molecule substrates. Chemistry & Biology, Vol 5, 669-678, November 1998
  17. ^ Unrau, P. J.; Bartel, D. P. RNA-catalysed nucleotide synthesis. Nature. 1998, 395 (6699): 260–263. Bibcode:1998Natur.395..260U. PMID 9751052. doi:10.1038/26193. 
  18. ^ Erives A. A Model of Proto-Anti-Codon RNA Enzymes Requiring L-Amino Acid Homochirality. J Molecular Evolution. 2011, 73 (1–2): 10–22. PMC 3223571 可免费查阅. PMID 21779963. doi:10.1007/s00239-011-9453-4. 
  19. ^ Zhang, Biliang; Cech, Thomas R. Peptide bond formation by in vitro selected ribozymes. Nature. 1997, 390 (6655): 96–100. Bibcode:1997Natur.390...96Z. PMID 9363898. doi:10.1038/36375. 
  20. ^ Szathmary, E. The origin of the genetic code: amino acids as cofactors in an RNA world. Trends in Genetics. 1999, 15 (6): 223–229. PMID 10354582. doi:10.1016/S0168-9525(99)01730-8. 
  21. ^ http://www.humpath.com/uracil
  22. ^ Lindahl, T. Instability and decay of the primary structure of DNA. Nature. April 1993, 362 (6422): 709–15. Bibcode:1993Natur.362..709L. PMID 8469282. doi:10.1038/362709a0. 
  23. ^ Pääbo, S. Ancient DNA. Scientific American. November 1993, 269 (5): 60–66. doi:10.1038/scientificamerican1193-86. 
  24. ^ Nudler E, Mironov AS. The riboswitch control of bacterial metabolism. Trends Biochem Sci. 2004, 29 (1): 11–7. PMID 14729327. doi:10.1016/j.tibs.2003.11.004. 
  25. ^ Tucker BJ, Breaker RR. Riboswitches as versatile gene control elements. Current Opinion in Structural Biology. 2005, 15 (3): 342–8. PMID 15919195. doi:10.1016/j.sbi.2005.05.003. 
  26. ^ Switching the light on plant riboswitches. Samuel Bocobza and Asaph Aharoni Trends in Plant Science Volume 13, Issue 10, October 2008, Pages 526-533 doi:10.1016/j.tplants.2008.07.004 PMID 18778966
  27. ^ Narberhaus F, Waldminghaus T, Chowdhury S. RNA thermometers. FEMS Microbiol. Rev. January 2006, 30 (1): 3–16 [2011-04-23]. PMID 16438677. doi:10.1111/j.1574-6976.2005.004.x. 
  28. ^ Patton, John T. Editor (2008). Segmented Double-stranded RNA Viruses: Structure and Molecular Biology. Caister Academic Press. Editor's affiliation: Laboratory of Infectious Diseases, NIAID, NIH, Bethesda, MD 20892-8026. ISBN 978-1-904455-21-9
  29. ^ Bell, Graham: The Basics of Selection. Springer, 1997.
  30. ^ Orgel, L. The origin of life on earth. Scientific American. 1994, 271 (4): 81. PMID 7524147. doi:10.1038/scientificamerican1094-76. 
  31. ^ Levy, Matthew; Miller, Stanley L. The stability of the RNA bases: Implications for the origin of life. PNAS. 1998, 95 (14): 7933–7938. Bibcode:1998PNAS...95.7933L. PMC 20907 可免费查阅. PMID 9653118. doi:10.1073/pnas.95.14.7933. 
  32. ^ Larralde, R.; Robertson, M. P.; Miller, S. L. Rates of decomposition of ribose and other sugars: implications for chemical evolution. PNAS. 1995, 92 (18): 8158–8160. Bibcode:1995PNAS...92.8158L. PMC 41115 可免费查阅. PMID 7667262. doi:10.1073/pnas.92.18.8158. 
  33. ^ Joyce GF; et al. Chiral selection in poly(C)-directed synthesis of oligo(G). Nature. 1984, 310 (5978): 602–604. Bibcode:1984Natur.310..602J. PMID 6462250. doi:10.1038/310602a0. 
  34. ^ Direct Assembly of Nucleoside Precursors from Two- and Three-Carbon Units Carole Anastasi, Michael A. Crowe, Matthew W. Powner, John D. Sutherland Angewandte Chemie International Edition Volume 45, Issue 37 , Pages 6176–79
  35. ^ Potentially Prebiotic Synthesis of Pyrimidine β-D-Ribonucleotides by Photoanomerization/Hydrolysis of α-D-Cytidine-2′-Phosphate Matthew W. Powner, John D. Sutherland ChemBioChem Volume 9, Issue 15 , Pages 2386–87
  36. ^ 36.0 36.1 Powner MW, Gerland B, Sutherland JD. Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions. Nature. 2009, 459 (7244): 239–242. Bibcode:2009Natur.459..239P. PMID 19444213. doi:10.1038/nature08013. 
  37. ^ Van Noorden R. RNA world easier to make. Nature. 2009. doi:10.1038/news.2009.471. 
  38. ^ Callahan, M.P.; Smith, K.E.; Cleaves, H.J.; Ruzicka, J.; Stern, J.C.; Glavin, D.P.; House, C.H.; Dworkin, J.P. Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases. Proceedings of the National Academy of Sciences (PNAS). 11 August 2011, 108 (34): 13995 [2011-08-15]. Bibcode:2011PNAS..10813995C. doi:10.1073/pnas.1106493108. 
  39. ^ Steigerwald, John. NASA Researchers: DNA Building Blocks Can Be Made in Space. NASA. 8 August 2011 [2011-08-10]. 
  40. ^ ScienceDaily Staff. DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests. ScienceDaily. 9 August 2011 [2011-08-09]. 
  41. ^ Than, Ker. Sugar Found In Space. National Geographic. August 29, 2012 [August 31, 2012]. 
  42. ^ Staff. Sweet! Astronomers spot sugar molecule near star. AP News. August 29, 2012 [August 31, 2012]. 
  43. ^ Jørgensen, J. K.; Favre, C.; Bisschop, S.; Bourke, T.; Dishoeck, E.; Schmalzl, M. Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA (PDF). The Astrophysical Journal Letters. eprint. 2012, 757: L4. Bibcode:2012ApJ...757L...4J. doi:10.1088/2041-8205/757/1/L4. 
  44. ^ 44.0 44.1 44.2 44.3 44.4 44.5 44.6 Gordon C. Mills, Dean Kenyon. The RNA World: A Critique. Access Research Network. [10 Sep 2011]. 
  45. ^ Schopf, J. William. Life's origin: the beginnings of biological evolution. University of California Press. 2002: 150. ISBN 0-520-23390-5. 
  46. ^ Prebiotic RNA chemistry: realising the molecular biologist's dream. Engineering and Physical Sciences Research Council. [10 Sep 2011]. 

外部連結

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