The origins of life on Earth is a mystery. Several hypotheses compete to explain how inert molecules present on the Earth's surface became “alive”. A prevalent hypothesis suggests that the molecules of RNA, used by cells to transmit genetic information, formed first and that DNA eventually formed from it after. A new study now challenges this view.
As reported in Nature Chemistry, RNA and DNA might have formed together. The two molecules are very similar but not identical. DNA is a double helix, while RNA is a single strand. RNA has a ribose sugar instead of a deoxyribose sugar. They can interact with each other, but researchers have struggled to convert one into the other without using enzymes produced by living organisms.
For this reason, the “RNA world” hypothesis took hold: RNA, through changes and increasing complexity, formed first and eventually led to DNA. However, researchers have now found a few chemical-reaction steps to convert some possible precursor molecules of RNA into possible precursor molecules of DNA.
“These new findings suggest that it may not be reasonable for chemists to be so heavily guided by the RNA World hypothesis in investigating the origins of life on Earth,” co-principal investigator Dr Ramanarayanan Krishnamurthy, associate professor of chemistry at Scripps Research, said in a statement.
RNA is simpler and more versatile compared to DNA. Many viruses use it to encode their genetic information, and all organisms use it in the production of proteins and the transmission of instructions encoded in the more stable DNA. The team suspects that if the two molecules arose together, they might have quickly ended up in the strict roles we see today.
“There is the beginning of a realization in the field that RNA and DNA could have been mixed together initially but later separated according to the things they do best,” Krishnamurthy added.
Their study is based on a compound known as thiouridine, which can be converted into two molecules that could be precursors of early DNA building blocks. While the study is certainly not the last word on the RNA world hypothesis, it’s likely to get more people thinking about other scenarios that are possible.