For more than three decades, scientists have hypothesized that the origins of life on Earth can be traced back to a series of chemical reactions that gave rise to self-replicating RNA, and that RNA then went on to evolve to create proteins and DNA separately at a later point in history.
However, a new study led by scientists at The Scripps Research Institute (TSRI) and published in the journal Angewandte Chemie challenged this so-called “RNA World hypothesis,” using newly discovered evidence suggesting that RNA and DNA may have developed simultaneously.
“Even if you believe in a RNA-only world, you have to believe in something that existed with RNA to help it move forward,” Ramanarayanan Krishnamurthy, senior author of the new study and an associate professor of chemistry at TSRI, explained Wednesday in a statement. “Why not think of RNA and DNA rising together, rather than trying to convert RNA to DNA by means of some fantastic chemistry at a prebiotic stage?”
The RNA World hypothesis argues that self-replicating molecules of ribonucleic acid formed through a series of chemical reactions, which evolved to create both proteins and enzymes that eventually led to the production of DNA and, ultimately, the evolution of complex organisms. This, Krishnamurthy and his colleagues claim, may not necessarily have been the case.
RNA, DNA may have evolved at the same time, study authors claim
If the RNA World theory is accurate, they said, many experts believe that there would have been several instances in which RNA nucleotides and DNA backbones intermingled to form strands of “heterogeneous” molecules – stepping stones in the transition to actual, full-blown DNA.
The problem with this, according to the TSRI team, is that their research revealed a significant loss of stability when RNA and DNA shared a common backbone. Such molecules are not able to hold together as well as either pure RNA or pure DNA, which would mean that their ability to contain genetic information and to reproduce would likely have become comprised.
“We were surprised to see a very deep drop in what we would call the ‘thermal stability,’” said Krishnamurthy. The reason for this, he and his fellow authors reported, appeared to be associated with differences between the structures of DNA sugar molecules and RNA sugar molecules. The study supports previous work indicating that the mixing of RNA and DNA resulted in the loss of nucleotide-binding aptamers.
Their findings suggest that hybrid RNA-DNA molecules would likely have died off in the RNA World, leaving behind the more stable pure RNA molecules. Currently, when RNA nucleobases become joined to DNA strands in error, sophisticated enzymes enter the picture and correct their mistake, but the study authors believe that these enzymes would not have existed during the time RNA and DNA initially came to be, meaning that any transition from the former to the latter may have been too difficult without some mechanism to keep them separated.
Krishnamurthy’s team proposes instead that RNA and DNA may have arisen together. If this were the case, DNA could have established its own homogeneous system before encountering RNA. RNA may have still evolved to produce DNA, they noted, but only after first coming in contact with the molecules and learning about the raw materials that comprised it.
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