Editing Mitochondrion (section)

=== Alternative genetic code ===
<div style="border: 1px solid grey; float:right; margin-right:0.6em; padding:0.5em">
{|class="wikitable"
|+Exceptions to the standard genetic code in mitochondria<ref name="Alberts-2005"/>
|-
!Organism!!Codon!!Standard!!Mitochondria
|-
|[[Mammal]]s
|AGA,&nbsp;AGG
|Arginine
|Stop&nbsp;codon
|-
|[[Invertebrate]]s
|AGA, AGG
|Arginine
|Serine
|-
|[[Fungus|Fungi]]
|CUA
|Leucine
|Threonine
|- valign="top"
|rowspan=2|All of the above
|AUA
|Isoleucine
|Methionine
|-
|UGA
|Stop codon
|Tryptophan
|}
</div>
While slight variations on the standard [[genetic code]] had been predicted earlier,<ref>{{cite journal |last1=Crick |first1=F.H.C. |last2=Orgel |first2=L.E. |title=Directed panspermia |journal=Icarus |date=July 1973 |volume=19 |issue=3 |pages=341–346 |doi=10.1016/0019-1035(73)90110-3 |bibcode=1973Icar...19..341C }}</ref> none was discovered until 1979, when researchers studying [[human mitochondrial genetics|human mitochondrial genes]] determined that they used an alternative code.<ref>{{cite journal | vauthors = Barrell BG, Bankier AT, Drouin J | title = A different genetic code in human mitochondria | journal = Nature | volume = 282 | issue = 5735 | pages = 189–194 | date = November 1979 | pmid = 226894 | doi = 10.1038/282189a0 | bibcode = 1979Natur.282..189B }}</ref> Nonetheless, the mitochondria of many other eukaryotes, including most plants, use the standard code.<ref name="Elzanowski-2019">{{Cite web|url=https://www.ncbi.nlm.nih.gov/Taxonomy/Utils/wprintgc.cgi|access-date=February 10, 2023|website=www.ncbi.nlm.nih.gov|title=The Genetic Codes|vauthors=Elzanowski A, Ostell J|date=January 7, 2019|archive-date=May 13, 2011|archive-url=https://web.archive.org/web/20110513014234/http://www.ncbi.nlm.nih.gov/Taxonomy/Utils/wprintgc.cgi|url-status=live}}</ref> Many slight variants have been discovered since,<ref name="Elzanowski-2019"/> including various alternative mitochondrial codes.<ref>{{cite journal | vauthors = Jukes TH, Osawa S | title = The genetic code in mitochondria and chloroplasts | journal = Experientia | volume = 46 | issue = 11–12 | pages = 1117–1126 | date = December 1990 | pmid = 2253709 | doi = 10.1007/BF01936921 }}</ref> Further, the AUA, AUC, and AUU codons are all allowable start codons.

Some of these differences should be regarded as pseudo-changes in the genetic code due to the phenomenon of [[RNA editing]], which is common in mitochondria. In higher plants, it was thought that CGG encoded for [[tryptophan]] and not [[arginine]]; however, the codon in the processed RNA was discovered to be the UGG codon, consistent with the standard [[genetic code]] for tryptophan.<ref>{{cite journal | vauthors = Hiesel R, Wissinger B, Schuster W, Brennicke A | title = RNA editing in plant mitochondria | journal = Science | volume = 246 | issue = 4937 | pages = 1632–1634 | date = December 1989 | pmid = 2480644 | doi = 10.1126/science.2480644 | bibcode = 1989Sci...246.1632H }}</ref> Of note, the arthropod mitochondrial genetic code has undergone parallel evolution within a phylum, with some organisms uniquely translating AGG to lysine.<ref>{{cite journal | vauthors = Abascal F, Posada D, Knight RD, Zardoya R | title = Parallel evolution of the genetic code in arthropod mitochondrial genomes | journal = PLOS Biology | volume = 4 | issue = 5 | pages = e127 | date = May 2006 | pmid = 16620150 | pmc = 1440934 | doi = 10.1371/journal.pbio.0040127 | doi-access = free }}</ref>