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Alien biochemistry
#1
Hi everyone, I'm new to the forum here. I just signed up today, even though I've been reading orions arm for several years. One topic that raised my interest was about the nature of alien biochemistry. Specifically, about organisms that rely on carbon for structure, and water as a solvent. (These are called type 1 life forms in the OA page) Is it inevitable that carbon based life would use DNA and/or RNA to encode genetic information? Are other arrangements possible, or even likely?

A related question: Are there any type 1 biospheres in the OA universe that use something other than DNA? Or that maybe use DNA with different nucleotides, something other than A-G-C-T? I'm trying to learn more about this and am grateful for any answers.
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#2
I'd guess that about a third of all biospheres in the Terragen Sphere are still in the prebiotic stage, and many of those do not have a dominant form of replicating molecule but are chemical soups of metabolising molecules without genetics. The majority of biospheres do use replicating molecules, however; many will still be in the RNA-world stage, and some of these might be highly sophisticated. Others would use DNA (either dextro- or levo-), but this is not the only possibility by a long chalk.

Below is an image from The Origins of the RNA World (Robertson and Foyce 2012) which gives some alternatives.
(A) RNA; (B) p-RNA; © TNA; (D) GNA; (E) PNA; (F) ANA; (G) diaminotriazine-tagged (left) and dioxo-5-aminopyrimidine-tagged (right) oligodipeptides; and (H) tPNA. (ANA is one I haven't heard of before, and it is formed from a racemic mixture, unlike DNA. So this might support some sort of racemic biochemistry which uses both left-handed and right-handed molecules).


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#3
I haven't had time to check, but is it not also possible that a DNA analogue could be used - one with the same basic structure but different bases chosen from the many available? There are many purines and pyrimidines, after all.

Or, another possibility: AFAIK the genetic code is completely arbitrary; is it possible that alien life uses DNA with the same bases as ours but a different coding scheme?

And finally, I much doubt that the amino acids we use are the only ones suitable for use in proteins. In fact, there are quite a lot of Earthly proteins that incorporate amino acids there is no code for - they are converted after they go into the protein. The best example is hydroxyproline, used in colossal amounts (it's one of the major components of collagen) but with no DNA code for it at the moment.
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#4
Quote:Is it possible that alien life uses DNA with the same bases as ours but a different coding scheme?
In the Terragen Sphere there are several worlds with biospheres that use DNA (including Seattle, a world described recently by Cray). However it is inevitable that such worlds use a different DNA coding system to Earth, since the chance against the same code occurring on two planets in the Visible Universe is very low indeed.
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#5
(10-27-2016, 05:54 PM)stevebowers Wrote: I'd guess that about a third of all biospheres in the Terragen Sphere are still in the prebiotic stage, and many of those do not have a dominant form of replicating molecule but are chemical soups of metabolising molecules without genetics. The majority of biospheres do use replicating molecules, however; many will still be in the RNA-world stage, and some of these might be highly sophisticated. Others would use DNA (either dextro- or levo-), but this is not the only possibility by a long chalk.

Excellent response. That brings a couple other questions to mind. Is it possible that the 'alien' DNA in these other worlds could have an opposite chirality from terran DNA? Or would right-handed amino acids and left-handed nucleotides make them into something altogether different? And of these different replicating molecules you mentioned (TNA, GNA, PNA, etc), do any of them have advantages compared to DNA? In the prebiotic stage of a biosphere, are they any more likely to evolve and dominate over DNA?
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#6
I did mention chirality in the post you quoted; there is some evidence that L-DNA and lefthanded biochemistry is favoured naturally, since some of the amino acids found in carbonaceous chondrite meteors have a bias towards left-handedness. But that might be a local phenomenon, possibly originating in our star's formation cloud or in the cluster we originated in. The conditions in other solar systems or clusters might favour right-handedness.
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