10-27-2016, 05:54 PM
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).
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).