RNA世界學說

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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[编辑]

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(DNARNA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]莱斯利·奥格尔英语Leslie Orgel[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·里奇英语Alexander Rich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[编辑]

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[编辑]

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:

  • 具有自我复制英语Self-replication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。One version, 189-bases long, had fidelity of 98.9%,[16] which would mean it would make an exact copy of an RNA molecule as long as itself in one of every eight copies. This 189 base pair ribozyme could polymerize a template of at most 14 nucleotides in length, which is too short for replication, but a potential lead for further investigation. The longest primer extension performed by a ribozyme polymerase was 20 bases.[17]
  • The ability to catalyze simple chemical reactions—which would enhance creation of molecules that are building blocks of RNA molecules (i.e., a strand of RNA which would make creating more strands of RNA easier). Relatively short RNA molecules with such abilities have been artificially formed in the lab.[18][19]
  • The ability to conjugate an amino acid to the 3'-end of an RNA in order to use its chemical groups or provide a long-branched aliphatic side-chain.[20]
  • The ability to catalyse the formation of peptide bonds to produce short peptides or longer proteins. This is done in modern cells by ribosomes, a complex of several RNA molecules known as rRNA together with many proteins. The rRNA molecules are thought responsible for its enzymatic activity, as no amino acid molecules lie within 18Å of the enzyme's active site.[7] A much shorter RNA molecule has been synthesized in the laboratory with the ability to form peptide bonds, and it has been suggested that rRNA has evolved from a similar molecule.[21] It has also been suggested that amino acids may have initially been involved with RNA molecules as cofactors enhancing or diversifying their enzymatic capabilities, before evolving to more complex peptides. Similarly, tRNA is suggested to have evolved from RNA molecules that began to catalyze amino acid transfer.[22]

參考文獻[编辑]

  1. ^ Woese, Carl. The Genetic Code. Harper & Row. 1968.Jan. ISBN 978-0060471767. 
  2. ^ Gilbert, Walter. The RNA World. Nature. 1986.Feb, 319: 618. doi:10.1038/319618a0. 
  3. ^ 引用错误:无效<ref>标签;未为name属性为Cech的引用提供文字
  4. ^ Crick FH. The origin of the genetic code. J Mol Biol. 1968, 38 (3): 367–379. doi:10.1016/0022-2836(68)90392-6. PMID 4887876. 
  5. ^ Orgel LE. Evolution of the genetic apparatus. J Mol Biol. 1968, 38 (3): 381–393. doi:10.1016/0022-2836(68)90393-8. PMID 5718557. 
  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 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. doi:10.1007/BF01732468. PMID 1263263. 
  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. doi:10.1126/science.290.5495.1306. PMID 11185405. 
  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. doi:10.1073/pnas.97.8.3868. PMC 18108. PMID 10760258. 
  12. ^ 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. doi:10.1038/nature08013. PMID 19444213. 
  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. doi:10.1002/cbdv.200790060. PMID 17443885. 
  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. doi:10.1016/0092-8674(87)90562-9. PMID 2436805. 
  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. doi:10.1126/science.1060786. PMID 11358999. 
  16. ^ W. K. Johnston, P. J. Unrau, M. S. Lawrence, M. E. Glasner and D. P. Bartel RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension. Science 292, 1319 (2001)
  17. ^ Hani S. Zaher and Peter J. Unrau, Selection of an improved RNA polymerase ribozyme with superior extension and fidelity. RNA (2007), 13:1017-1026
  18. ^ Huang, Yang, and Yarus, RNA enzymes with two small-molecule substrates. Chemistry & Biology, Vol 5, 669-678, November 1998
  19. ^ Unrau, P. J.; Bartel, D. P. RNA-catalysed nucleotide synthesis. Nature. 1998, 395 (6699): 260–263. Bibcode:1998Natur.395..260U. doi:10.1038/26193. PMID 9751052. 
  20. ^ Erives A. A Model of Proto-Anti-Codon RNA Enzymes Requiring L-Amino Acid Homochirality. J Molecular Evolution. 2011, 73 (1–2): 10–22. doi:10.1007/s00239-011-9453-4. PMC 3223571. PMID 21779963. 
  21. ^ Zhang, Biliang; Cech, Thomas R. Peptide bond formation by in vitro selected ribozymes. Nature. 1997, 390 (6655): 96–100. Bibcode:1997Natur.390...96Z. doi:10.1038/36375. PMID 9363898. 
  22. ^ Szathmary, E. The origin of the genetic code: amino acids as cofactors in an RNA world. Trends in Genetics. 1999, 15 (6): 223–229. doi:10.1016/S0168-9525(99)01730-8. PMID 10354582. 

外部連結[编辑]