Scientists at the Scripps Research Institute in La Jolla, California, think they have a new take on an old tale about how life began here on Earth, over four-billion years ago.
The story goes that only half a billion years after our planet was formed, life began to emerge from the primordial soup of chemicals in the form of RNA, which served as the molecular starting point for the creation of amino acids, proteins and, eventually, cellular organisms.
According to the theory, RNA molecules came first -before DNA, most importantly- because of their ability to be self-replicating. Scientists have discovered that some RNA molecules called ribozymes have the power to build nucleotides from parts in their surrounding environment and can then pair these up to form new, replica RNAs. Thus, the “RNA world” theory claims that even though DNA molecules are now known to be the blueprint for all living things, it was nonetheless self-replicating RNA that started the ball rolling all those years ago.
Although accepted for the most part by the scientific community, questions nonetheless abound concerning the RNA world hypothesis, one of which focuses on the presumed transition stage between the evolution of RNA and the subsequent emergence of DNA. This stage is said to have featured hybrid RNA-DNA formations -pieces of DNA, for example, fitted with RNA nucleotides- called “heterogeneous strands.”
But a new study is calling into doubt the possibility that nature could have produced such a thing as viable heterogeneous strands. Researchers found that when DNA and RNA are put together into a hybrid formation, the result is not stable enough to replicate.
“We were surprised to see a very deep drop in what we would call the ‘thermal stability,’” says Ramanarayanan Krishnamurthy, associate professor of chemistry at the Scripps Research Institute and senior author of the study.
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Krishnamurthy and his colleagues believe that these problems in the creation of heterogeneous strands point out the flaw in the RNA world theory and actually give credence to an alternative hypothesis, namely, that DNA and RNA molecules evolved at the same time.
“Even if you believe in a RNA-only world, you have to believe in something that existed with RNA to help it move forward,” said Krishnamurthy. “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 new study comes at a time when genetic research is reaching a feverish pitch, thanks to the invention of gene editing tool CRISPR-Cas9, which allows for precise and effective manipulation of cellular DNA and has taken the scientific community by storm since its introduction only a few years ago. CRISPR holds a wealth of promise, from further improving crop yields on agricultural products (agribusiness giant Monsanto recently bought the rights to use CRISPR technology in an nonexclusive agreement) to eliminating debilitating genetic conditions to halting cancer through gene therapy. The journal Nature announced in July that Chinese scientists were already embarking on the first human trials using CRISPR in cancer treatment.
“That’s fast,” said Dr. Jason Moffat, molecular geneticist at the University of Toronto, in conversation with the CBC. “They’re pushing the technology really hard.”
This year the Gairdner Foundation gave the bulk of its Canada Gairdner International Awards in medical research to pioneers in the development of CRISPR-Cas9.