The two nucleic acids DNA and RNA are named for the type of sugar complex that forms each molecule’s sugar-phosphate backbone – a kind of molecular thread holding the nucleotide beads together.Could a simpler, self-replicating molecule have existed as a precursor to RNA, perhaps providing genetic material for earth’s earliest organisms?These chemical candidates are attractive to those seeking to unlock the still-elusive secret of how the first life began, as primitive molecular forms may have more readily emerged during the planet’s prebiotic era.One approach to identifying molecules that may have acted as genetic precursors to RNA and DNA is to examine other nucleic acids that differ slightly in their chemical composition, yet still possess critical properties of self-assembly and replication as well as the ability to fold into shapes useful for biological function.In an article released online in the journal Nature Chemistry, Chaput and his group describe the Darwinian evolution of functional TNA molecules from a large pool of random sequences.This is the first case where such methods have been applied to molecules other than DNA and RNA, or very close structural analogues thereof.With the aid of specialized enzymes known as polymerases, RNA assembles amino acids to form essential proteins.Remarkably, the basic functioning of the genetic code remains the same, whether the organism is a snail or a senator, pointing to a common ancestor in the DNA-based microbial life already flourishing some 3.5 billion years ago.
Once random DNA libraries were built excluding guanine, a high yield of DNA-TNA hybrid strands was produced.
They are simpler than the five-carbon pentose sugars found in both DNA and RNA and could assemble more easily in a prebiotic world, from two identical two-carbon fragments.
This advantage in structural simplicity was originally thought to be an Achilles’ heel for TNA, making its binding behavior incompatible with DNA and RNA.
Nevertheless, such ancestors were by this time quite complex, leading some scientists to speculate about still earlier forms of self-replication.
Before DNA emerged to play its dominant role as the design blueprint for life, a simpler genetic world dominated by RNA may have prevailed.
Once such a pool of TNA strands has been generated, a process of selection must successfully identify members that can perform a given function, excluding the rest.