Biologists from the United States have proven the likelihood of an alternative variant of the evolution of life, according to which RNA and DNA molecules arose simultaneously, bypassing the stage of the RNA world, according to an article published in the journal Angewandte Chemie.
“Even if you believe in the 'world of RNA', you have to take on faith and the position that something existed simultaneously with RNA and helped life to evolve towards DNA and proteins. Why not assume that DNA and RNA arose at the same time, and not try to turn the second into the first with some crazy chemistry,”said Ramanarayanan Krishnamurthy of the Scripps Institute in La Jolla (USA).
According to the generally accepted theory of the "RNA-world" today, the role of proteins and DNA in the first living organisms on earth was played by RNA molecules. Subsequently, proteins began to control cellular processes, and DNA took the role of a repository of genetic information. Scientists are now experimenting with short RNA molecules in an attempt to replicate the origins of life in a laboratory.
In particular, molecular evolutionary biologists today are trying to understand how RNA molecules were replaced by their DNA counterparts. Most scientists assume that this happened gradually - first, the so-called "chimeras" appeared, molecules consisting of links of both RNA and DNA, and then RNA nucleotides were gradually replaced by the familiar "letters" contained in our genetic code.
Krishnamurti and his colleagues found that this most likely could not have happened in reality, having studied how stable such chimeric "hybrids" of DNA and RNA would be. To do this, scientists have collected several short chains of "pure" DNA and RNA molecules, composed of 16, 10 and 6 "letters" -nucleotides.
Scientists have processed some of these molecules in such a way that some of the "letters" in them turned into analogs used in RNA or DNA, as a result of which they became chimeric "hybrids", similar to those "building blocks of life" that existed during the origin of life. Having prepared solutions of such "chimeras", the authors of the article heated them up and tracked how quickly they decomposed.
As it turned out, even the addition of one foreign "letter" led to a sharp decrease in the stability of the molecules, reducing the temperature of its decomposition by 10-20 degrees Celsius. A similar reaction was observed both in DNA molecules and in RNA molecules with changes in any types of letters and the bonds between them, which suggests that all chimeras are extremely unstable in nature.
Their instability, as Krishnamurti admits, does not mean that they could not exist in principle in the primary "soup" of the Earth's oceans, but their existence in the first living cells would require the presence of complex enzymes that would constantly "repair" such molecules and protect them from decay. It is highly unlikely that they arose before the first proteins and DNA appeared.
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Therefore, according to Krishnamurti, a smooth transition from the RNA world to modern life was impossible, which prompted him to think that both RNA and DNA arose at the same time, and that the first life could use both of them in its life. Such a scenario, in his opinion, is more consistent with the results of experiments and what we see in modern organisms than the hypothesis of the "RNA world".