Scripps research team resurrect hypothesis that RNA and DNA co-evolved

A research team at The Scripps Research Institute in La Jolla, California has demonstrated that in the presence of two chemical compounds DAP and AIM, DNA building blocks are phosphorylated and oligomerize in the canonical orientation of DNA. This discovery brings some new ideas about how life might have evolved on Earth.

The scientists carried out a series of chemical experiments to demonstrate that DNA oligomerization without enzymes is possible. They say that the necessary conditions are similar to the necessary conditions for RNA to exist.

The work was published in the chemistry journal Angewandte Chemie on Dec. 15, 2020.

The predominant hypothesis until now has been that RNA emerged as a chemical precursor to life on Earth, without the presence of DNA, and that DNA developed later as a result of RNA building blocks. The so-called "RNA world hypothesis" has been the subject of many investigations over the years, but the researchers say, "it must be pointed the RNA-centric view and the RNA-only approach is limited in its validity and has been rightfully questions critically."

To provide evidence for RNA and DNA co-evolving, the Scripps group first carried out experiments to show that DNA oligomerization is possible under a single set of conditions without an enzyme. Their reactions did require diamidophosphate (DAP). The team showed that this compound and AIM allow DNA bases to bond end-to-end.

DAP directed phosphorylation (the addition of a phosphate to an organic compound) of specific pyrimidine deoxynucleosides to form pyrimidine 5'-O-amidophosphates. These molecules then further react to form short DNA oligomers in the canonical 3',5'-phosohdiester DNA bond pattern.

The researchers discovered some fascinating chemical behavior with their experiments. The pyrimidines (C and T) were more efficiently phosphorylated than the purines (A and G). But, when the complementary pyrimidines were included with the purines the efficiency of producing purines increased.

“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,” the first author of the study, Eddy Jiménez, told the Scripps Research newsroom.

They speculated that this may be due to hydrogen bond assistance.

The next step of the research group will be to examine mixes of RNA and DNA to see what kinds of molecules form and can evolve. 

“This finding is an important step toward the development of a detailed chemical model of how the first life forms originated on Earth,” senior author Ramanarayanan Krishnamurthy, an associate professor of chemistry at Scripps Research, told the Scripps news office.