Genetic code: Difference between revisions

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==Origin==

The genetic code is a key part of the [[origin of life|history of life]],. accordingUnder tothe one[[RNA versionworld of whichhypothesis]], self-replicating RNA molecules preceded lifesignificant asuse weof know itproteins. This isUnder the [[RNAnucleopeptide world hypothesis]]. Under this hypothesis, anysignificant modeluse forof thepeptides emergence ofpreceded the genetic code isand intimatelywas relatedconcurrent towith life's asophisticated modeluse of theRNA.<ref>{{cite transferjournal from|last1=Fried [[ribozyme]]s|first1=Stephen (RNAD. enzymes)|last2=Fujishima to|first2=Kosuke proteins|last3=Makarov as|first3=Mikhail the|last4=Cherepashuk principal|first4=Ivan enzymes|last5=Hlouchova in|first5=Klara cells.|title=Peptides Inbefore lineand withduring the RNAnucleotide world: hypothesis,an origins story emphasizing cooperation between proteins and nucleic acids |journal=Journal of The Royal Society Interface |date=February 2022 |volume=19 |issue=187 |doi=doi:10.1098/rsif.2021.0641}}</ref> transferTransfer RNA molecules appear to have evolved before modern [[aminoacyl-tRNA synthetase]]s, so the latter cannot be part of the explanation of its patterns.<ref name=De1998>{{cite journal | vauthors = Ribas de Pouplana L, Turner RJ, Steer BA, Schimmel P | title = Genetic code origins: tRNAs older than their synthetases? | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 95 | issue = 19 | pages = 11295–300 | date = Sep 1998 | pmid = 9736730 | pmc = 21636 | doi = 10.1073/pnas.95.19.11295 | bibcode = 1998PNAS...9511295D | doi-access = free }}</ref>. It is possible that synthetases replaced an earlier system of [[ribozyme]]s (RNA enzymes), or that amino acids might were recognized by unique pockets in the tertiary structure of proto-tRNAs.<ref>{{cite journal |last1=Koonin |first1=Eugene |title=Frozen Accident Pushing 50: Stereochemistry, Expansion, and Chance in the Evolution of the Genetic Code |journal=Life |date=23 May 2017 |volume=7 |issue=2 |pages=22 |doi=10.3390/life7020022}}</ref>

A hypothetical randomly evolved genetic code further motivates a biochemical or evolutionary model for its origin. If amino acids were randomly assigned to triplet codons, there would be 1.5&nbsp;×&nbsp;10⁸⁴ possible genetic codes.<ref name="isbn0-674-05075-4">{{cite book|first=Michael |last=Yarus|author-link=Michael Yarus|title=Life from an RNA World: The Ancestor Within|url={{google books |plainurl=y |id=-YLBMmJE1WwC}}|year=2010|publisher=Harvard University Press|isbn=978-0-674-05075-4}}</ref>{{rp|[{{google books |plainurl=y |id=-YLBMmJE1WwC|page=163}} 163]}} This number is found by calculating the number of ways that 21 items (20 amino acids plus one stop) can be placed in 64 bins, wherein each item is used at least once.<ref>{{Cite web|url=http://community.wolfram.com/groups/-/m/t/319970|title=Mathematica function for # possible arrangements of items in bins? – Online Technical Discussion Groups—Wolfram Community|website=community.wolfram.com|language=en-US|access-date=2017-02-03}}</ref> However, the distribution of codon assignments in the genetic code is nonrandom.<ref name="pmid9732450">{{cite journal | vauthors = Freeland SJ, Hurst LD | s2cid = 20130470 | title = The genetic code is one in a million | journal = Journal of Molecular Evolution | volume = 47 | issue = 3 | pages = 238–48 | date = Sep 1998 | pmid = 9732450 | doi = 10.1007/PL00006381 | bibcode = 1998JMolE..47..238F }}</ref> In particular, the genetic code clusters certain amino acid assignments.