For a long time, the “RNA World” hypothesis has been widely accepted by chemists and molecular biologists as to how life on Earth arose. First proposed by Alexander Rich in 1962, this hypothesis suggests that primordial self-replicating RNA arose before proteins and DNA. However, there have been recent studies contradicting this hypothesis, proposing that RNA and DNA may have in fact formed together.
A new study published in the journal Angewandte Chemie has joined this opposition, suggesting that RNA and DNA originated together from similar chemical reactions and that the first self-replicating molecules may have been a DNA/RNA mixture. “This finding is an important step toward the development of a detailed chemical model of how the first life forms originated on Earth,” Dr Ramanarayanan Krishnamurthy, associate professor of chemistry at Scripps Research Institute and senior author of the study, said in a statement.
The authors of the paper state that RNA might be too “sticky” to have been the first self-replicating molecules. RNA strands replicate by one strand serving as a template for a complementary strand, which in present organisms is separated from the template by enzymes. However, RNA strands are not good at separating by themselves, and enzymes are proteins and therefore would not have existed in the “RNA World”. The researchers claim that “chimeric” strands, made of both RNA and DNA, could circumvent this problem by being less sticky.
In the study, the researchers built on previous studies of RNA and DNA formation in prebiotic (before life) conditions. These conditions do not include chemicals that only occur due to living organisms, only abiotic ones. This allows researchers to assess how life could have arisen from these abiotic conditions. This study focused on nucleosides – the building blocks of RNA and DNA – in the presence of the organic compounds 2-aminoimidazole and Diamidophosphate (DAP).
It was observed that, with both of these chemicals, deoxynucleosides (which make up DNA) reacted to produce short oligomers of DNA. Preliminary data indicated that the same occurred for ribonucleosides, which make up RNA. In a 2017 study, Krishnamurthy and colleagues showed that DAP could have played a key role in modifying ribonucleosides to string them together into the first RNA strands. The new study found under similar conditions it may do the same for DNA.
“We found, to our surprise, that using DAP to react with deoxynucleosides works better when the deoxynucleosides are not all the same but are instead mixes of different DNA 'letters' such as A and T, or G and C, like real DNA,” explained first author Dr Eddy Jiménez.
These results could give us more clarity as to how life on Earth originated, as well as being useful for research and industry. Many processes – such as polymerase chain reaction (PCR), used in testing for COVID-19 – rely on the artificial synthesis of DNA and RNA, but are reliant on often fragile enzymes. These findings could lead to enzyme-free alternative methods.