Editing Genetic code (section)

=== Non-standard amino acids ===
In some proteins, non-standard amino acids are substituted for standard stop codons, depending on associated signal sequences in the messenger RNA. For example, UGA can code for [[selenocysteine]] and UAG can code for [[pyrrolysine]]. Selenocysteine came to be seen as the 21st amino acid, and pyrrolysine as the 22nd.<ref name=Zhang2005/> Both selenocysteine and pyrrolysine may be present in the same organism.<ref name=Zhang2005>{{cite journal | vauthors = Zhang Y, Baranov PV, Atkins JF, Gladyshev VN | title = Pyrrolysine and selenocysteine use dissimilar decoding strategies | journal = The Journal of Biological Chemistry | volume = 280 | issue = 21 | pages = 20740–51 | date = May 2005 | pmid = 15788401 | doi = 10.1074/jbc.M501458200 | url = http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1071&context=biochemgladyshev | doi-access = free }}</ref> Although the genetic code is normally fixed in an organism, the achaeal prokaryote ''[[Acetohalobium arabaticum]]'' can expand its genetic code from 20 to 21 amino acids (by including pyrrolysine) under different conditions of growth.<ref name=Prat2012>{{cite journal | vauthors = Prat L, Heinemann IU, Aerni HR, Rinehart J, O'Donoghue P, Söll D | title = Carbon source-dependent expansion of the genetic code in bacteria | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 51 | pages = 21070–5 | date = Dec 2012 | pmid = 23185002 | pmc = 3529041 | doi = 10.1073/pnas.1218613110 | bibcode = 2012PNAS..10921070P | doi-access = free }}</ref>