Genetic code: Difference between revisions

Revision as of 00:11, 19 July 2025

Revision as of 09:33, 19 July 2025

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==Origin==

The genetic code is a key part of the [[origin of life|history of life]]. Under the [[RNA world hypothesis]], self-replicating RNA molecules preceded significant use of proteins. Under the nucleopeptide world hypothesis, significant use of peptides preceded the genetic code and was concurrent with early life's sophisticated use of RNA.<ref>{{cite journal |last1=Fried |first1=Stephen D. |last2=Fujishima |first2=Kosuke |last3=Makarov |first3=Mikhail |last4=Cherepashuk |first4=Ivan |last5=Hlouchova |first5=Klara |title=Peptides before and during the nucleotide world: 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=10.1098/rsif.2021.0641 |pmid=35135297 |pmc=8833103 }}</ref> Transfer RNA molecules appear to have evolved before modern [[aminoacyl-tRNA synthetase]]s.<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 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 |pmid=28545255 |bibcode=2017Life....7...22K |doi-access=free }}</ref> It is not known why the genetic code only uses L-amino acids and not D-amino acids.<ref name="pmid21779963">{{cite journal | vauthors = Erives A | title = A model of proto-anti-codon RNA enzymes requiring L-amino acid homochirality | journal = Journal of Molecular Evolution | volume = 73 | issue = 1–2 | pages = 10–22 | date = Aug 2011 | pmid = 21779963 | doi = 10.1007/s00239-011-9453-4 | pmc=3223571| bibcode = 2011JMolE..73...10E }}</ref>

Any evolutionary model for the code's origin must account for its [[Robustness (evolution)|robustness]] of encoded proteins to errors during DNA replication and during translation. Many single nucleotide errors are [[Synonymous substitution|synonymous]], and those that are not tend to cause the [[Conservative replacement|substitution of a biochemically similar amino acid]]. Even holding the structure of the code the same such that clusters of codons encode the same amino acid, which amino acids are encoded by which sets of codons is "one in a million" with respect to robustness.<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> Biochemically similar amino acids tend to share the same middle nucleotide, while synonymous changes generally happen at the third nucleotide.