Genome complexity: different DNA used (Introduction)

by David Turell , Friday, November 12, 2021, 15:31 (895 days ago) @ David Turell

Some organisms use an entirely different form of DNA:

https://www.the-scientist.com/news-opinion/screen-of-250-000-species-reveals-tweaks-to-...

"A massive screen of bacterial and archaeal genomes revealed five previously unknown instances where an organism uses an alternate code to translate genetic blueprints into proteins.

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"Prior to the new study, all known alternate genetic codes in bacteria involved changing a “stop” codon that instructs cellular machinery that it’s reached the end of a protein into one associated with an amino acid. The new results reveal the first alternate genetic codes in bacteria that represent a sense codon reassignment—that is, changing a codon from one amino acid to another.

"Identifying these alternate codes is important, Eddy says, because scientists who predict what proteins an organism will synthesize based on its genome do so under the assumption that the standard genetic code is at play. Accounting for deviations, then, will improve the accuracy of those predictions and prevent errors from being codified into databases as more and more genomes are sequenced.

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"Part of the reason changes do happen, Shulgina explains, is that some bacterial genomes may have a low composition of certain nucleotides compared to others. That brings the usage of codons that rely on those nucleotides down to nearly zero, making it easier for an organism to survive shifts without altering too many proteins in a drastic way.

“'At least in bacteria, it seems like these sorts of forces might explain why the genetic code evolved this way,” Shulgina says. “This might be totally different if we looked in other forms of life like eukaryotes.” Exceptions to the standard genetic code have been found in single-celled eukaryotes such as yeast, but experts expect alternate codes to be rare in more complex eukaryotic organisms.

"Tracing down why these alternate genetic codes emerged during evolutionary history is difficult, multiple researchers tell The Scientist, in no small part because humans couldn’t watch it happen. But the authors do have some hypotheses.

"In one case, Shulgina identified a bacterium that uses the same alternate code as a bacteriophage virus that infects it, indicating that the bacteria seemingly evolved an alternate code that prevented its cellular machinery from being hijacked—and that the phage may have then made the same adaptation to follow its host."

Comment: So DNA is not absolutely set in stone in simple organisms. No studies have been done on more advanced complex eukaryotes.