RNA is a versatile molecule that can act as a transmitter of genetic information, as a catalyst for chemical reactions (ribozymes), including its own synthesis.
The “RNA world” hypothesis is one of the pillars of research on the origin of life. According to her, before life as we know it, RNA molecules must have been formed that assumed, in addition to their current functions, those of DNA and enzymes. An interesting recent study published in the journal Nature goes one step further.
The article shows that a simple RNA can generate peptides under the conditions of the “RNA world”. Thus, it would act as a very simple predecessor of the function carried out by the ribosome. The chemistry shows that the abiotic formation of an RNA-peptide hybrid is possible.
The interest of this work lies in its coherence with a global model. For those of us who have worked on the previous steps in the origin of life process, it is consistent with the role of urea as a key compound in the origin of life. The work adds another one to the multiple roles of this molecule in the prebiotic world: urea as a connector in a primordial peptide synthesis.
It is also consistent with what we know about molecular biology. Thus, this work is not a paradigm shift, but clarifies a historical misunderstanding regarding the meaning of the concept “RNA world”.
Canonical, non-canonical nucleotides and RNA paradoxes
The bases of DNA are adenine, guanine, cytosine, and thymine. The bases of RNA are adenine, guanine, cytosine, and uracil. We call them “canon bases”. They are the majority in the structure of nucleic acids and responsible for the language of the genetic code.
It is tempting to focus on canonical bases (and their corresponding nucleotides) when thinking about the origin of life.
RNA is a versatile molecule that can act as a transmitter of genetic information, as a catalyst for chemical reactions (ribozymes), including its own synthesis. The reader will think that, perhaps, life was preceded by the formation of RNA molecules capable of generating copies of themselves and starting life (track 1 in the diagram). It seems logical that these molecules opened the way to the origin of proteins and metabolism. Thus, many scientists set out to find ways by which RNA could be generated before life, from its canonical components.
However, RNA is paradoxical.
For one thing, we think it’s so old that there was an “RNA world” before cellular life.
On the other hand, it is very difficult for an RNA molecule to be formed in the prebiotic world.
One property of canonical bases is that they resist forming nucleotides and connecting them, forming RNA. Despite recent advances, we cannot give a clear answer to this problem and perhaps route 1 of the scheme is not the right way to go. In fact, the canonical bases have been found in meteorite samples . But there is no clue in them that points to RNA; the RNA paradox makes it unlikely that life came from space.
In addition to these difficulties in generating RNA in the prebiotic world, there is a selection problem: why do DNA and RNA have this composition? How were the canonical bases chosen ?
We are not sure.
In the prebiotic world, the canonical bases must have arisen together with other molecules that can carry out the same function, even more easily: they are the non-canonical bases and nucleotides . In a problem called the paradox of base pairing we see that, precisely, the difficult thing is to select the canonical bases among all the others.
What clues does life itself give us?
One is that RNA requires the presence of non-canonical nucleotides for its function. The high school adage was incomplete: the composition of RNA is much more complex and includes dozens of exotic bases. Non-canonical nucleotides remain essential in life today and are key to evolution. Perhaps, at the origin of life, there was a similar molecule, prior to the current RNA, in which exotic components were even more important and which evolved, keeping only the necessary non-canonical components (path 2: preRNA).
The current RNA would be a biological product, not a prebiotic, the result of the evolution of the genetic code, keeping the non-canonical components in the form of base modifications.
Biology also teaches us that RNA and proteins always go together. Both interact, complement each other and need each other. We know that the function of RNA as a generator of proteins is very old, perhaps prior to life itself. It is reasonable to think that both types of molecules coexisted (pathways 3 and 4).
This latest study shows that, chemically, this co-evolution pathway is possible in a way that preserves the logic of life as we know it: non-canonical nucleotides essential for RNA function and their action in peptide synthesis as a primary function. , paving the way to the oldest and best-preserved common structure of terrestrial life: the ribosome.
The concept of the “RNA world” has been misunderstood at times. As both the chemistry and the RNA of current life teach us, it is the set of canonical, non-canonical and peptide components that made possible a path of chemical evolution prior to biological evolution. The “RNA world” is a preRNA-peptide world .
Fortunately, all of this is still an open question; We still have a lot to learn, at an exciting time for the knowledge of the origin of life on our planet.