The leading explanation for how life started on Earth is the RNA world hypothesis. The theory posits that life later evolved to use DNA because of the relative instability and poor catalytic properties of RNA. RNA came first and evolved into DNA that eventually went on to form complex life.
Findings of a new study, however, offer evidence suggesting that RNA would not have been able to sustainably give rise to DNA.
The research conducted by chemists from The Scripps Research Institute in California leads researchers to suggest a second alternative RNA-DNA origin story that proposes the two molecules may have formed at the same time.
DNA, whose structure looks like a ladder with nucleobase pairs as the rungs and the sugar molecules forming the side, appears like a completed version of the RNA, which looks like just one side of the ladder.
If the RNA world theory is correct, there would have been an intermediary stage that created heterogenous strands characterized by RNA nucleotides serving as the rungs and the DNA sugar molecules as the side.
Ramanarayanan Krishnamurthy, an associate professor of chemistry at TSRI, and colleagues built these so-called DNA-RNA chimeras and found instability problems. The chimeras do not stay together as pure DNA or pure RNA compromising the molecules' ability to hold genetic information and replicate.
The instability may have been caused by a difference in the RNA sugar molecule and DNA sugar molecule structures. Because of this instability, the chimeras in the RNA world may have died off in favor of the more stable RNA molecules.
In cells today, sophisticated enzymes will make a quick fix if RNA nucleobases mistakenly join a DNA strand. Evolution favors more stable, homogeneous molecules. Researchers said these enzymes have not yet likely existed during the early evolution of the RNA and DNA so the substitution may have crippled the molecules' ability to replicate and function.
The findings led researchers to propose an alternative theory that suggests RNA and DNA may have risen in tandem.
"These results point to the difficulties for the transition from one homogeneous system (RNA) to another (RNA/DNA) in an RNA world with a heterogeneous mixture of ribo- and deoxyribonucleotides and sequences, while suggesting an alternative scenario of prebiological accumulation and co-evolution of homogeneous systems (RNA and DNA)," the researchers wrote in their study, which was published in Angewandte Chemie on Sept. 21.
Krishnamurth and colleagues were not the first to propose this theory but their findings offer scientists new evidence that could strengthen what Canadian American biologist Jack Szostak of Harvard University has already demonstrated. The 63-year-old Nobel laureate has shown there is loss of function when RNA and DNA are mixed.