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The RNA world refers to the self-replicating ribonucleic acid (RNA) molecules that were precursors to all current life on Earth. It is generally accepted that current life on Earth descends from an RNA world,〔 * "The proposal that life on Earth arose from an RNA World is widely accepted." * "It now seems very likely that our familiar DNA/RNA/protein world was preceded by an RNA world" * "There is now strong evidence indicating that an RNA World did indeed exist before DNA- and protein-based life." *"(RNA world's existence ) has broad support within the community today."〕 although RNA-based life may not have been the first life to exist.〔 The RNA world would have eventually been replaced by the DNA, RNA and protein world of today, likely through an intermediate stage of ribonucleoprotein enzymes such as the ribosome and ribozymes, since proteins large enough to self-fold and have useful activities would only have come about after RNA was available to catalyze peptide ligation or amino acid polymerization.〔Cech, T.R. (2011). The RNA Worlds in Context. Source: Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215. Cold Spring Harb Perspect Biol. 2011 Feb 16. pii: cshperspect.a006742v1. . (ahead of print )〕 DNA is thought to have taken over the role of data storage due to its increased stability, while proteins, through a greater variety of monomers (amino acids), replaced RNA's role in specialized biocatalysis. The RNA world hypothesis is supported by many independent lines of evidence, such as the observations that RNA is central to the translation process and that small RNAs can catalyze all of the chemical group and information transfers required for life.〔 The structure of the ribosome has been called the "smoking gun," as it showed that the ribosome is a ribozyme, with a central core of RNA and no amino acid side chains within 18 angstroms of the active site where peptide bond formation is catalyzed. Many of the most critical components of cells (those that evolve the slowest) are composed mostly or entirely of RNA. Also, many critical cofactors (ATP, Acetyl-CoA, NADH, etc.) are either nucleotides or substances clearly related to them. This would mean that the RNA and nucleotide cofactors in modern cells are an evolutionary remnant of an RNA-based enzymatic system that preceded the protein-based one seen in all extant life. == History == One of the challenges in studying abiogenesis is that the system of reproduction and metabolism utilized by all extant life involves three distinct types of interdependent macromolecules (DNA, RNA, and protein). This suggests that life could not have arisen in its current form, and mechanisms have then been sought whereby the current system might have arisen from a simpler precursor system. The concept of RNA as a primordial molecule〔 can be found in papers by Francis Crick and Leslie Orgel, as well as in Carl Woese's 1967 book ''The Genetic Code''.〔Woese C.R. (1967). The genetic code: The molecular basis for genetic expression. p. 186. Harper & Row〕 In 1962 the molecular biologist Alexander Rich, of the Massachusetts Institute of Technology, had posited much the same idea in an article he contributed to a volume issued in honor of Nobel-laureate physiologist Albert Szent-Györgyi. Hans Kuhn in 1972 laid out a possible process by which the modern genetic system might have arisen from a nucleotide-based precursor, and this led Harold White in 1976 to observe that many of the cofactors essential for enzymatic function are either nucleotides or could have been derived from nucleotides. He proposed that these nucleotide cofactors represent "fossils of nucleic acid enzymes". The phrase "RNA World" was first used by Nobel laureate Walter Gilbert in 1986, in a commentary on how recent observations of the catalytic properties of various forms of RNA fit with this hypothesis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「RNA world」の詳細全文を読む スポンサード リンク
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