In the great quest to uncover the secrets to the origin of life on Earth — a discipline referred to as biogenesis theory — there remains a profound (unanswered) question: how do “living” molecules arise from non-living ones? For, to answer such a question means making a distinction between different states of molecular organization. The question is almost metaphysical in nature — for it requires one to ponder the very definition of ‘Life’; it is nearly impossible to pinpoint a particular stage (or molecular structure) in the genesis of living matter and say this is now “alive”, when previously, it was “non-living” (e.g., Is it only the cell that is alive? What about mitochondria which also have DNA? What about the ribosomes that translate RNA messages into proteins?, etc.)
But there are a few requirements for living matter that scientists do recognize; such living molecules must possess two fundamental features: they must be capable of “self-organization” (also called “self-assembly”) under the right environmental conditions, and, they must be capable of self-replication. This last feature is tricky, since it implies that there is some form of “molecular memory”, or information coding, at work, such that these special molecules can remember how to copy themselves.
The RNA World hypothesis
The most promising candidate molecules here are RNA (ribonucleic acid) molecules which are both simpler than DNA molecules and more powerful catalysts for molecular interactions. Indeed, RNA-based enzymes (i.e., the RNA polymerases) are necessary for DNA replication. Many biologists believe that some form of RNA — or precursors to RNA — had to evolve first, before the more complex DNA molecule could appear. This view is known as the ‘RNA World’ hypothesis.
But, there’s one big problem with the RNA World hypothesis; the building blocks of RNA — the nucleoside bases cytosine (C), guanine (G), adenine (A) and urasil (U) — do not normally assemble themselves into longer chains to form the genetic structures we associate with living molecules.
This has led many scientists to posit precursors to these RNA bases — proto-RNAs — that must have evolved first. According to Nicholas Hud, a chemist at the Georgia Institute of Technology (Georgia Tech) in Atlanta, “RNA is so perfect today that it has to be the product of evolution”
The Experiments with “Proto-RNA” Molecules
To test this theory, Georgia Tech researchers, collaborating with colleagues from the Institute for Research in Biomedicine in Barcelona, Spain, began a series of experiments with two key proto-RNA candidate molecules: cyanuric acid (CA) and triaminopyrimidine (TAP). The latter molecule belongs to a family of chemicals called pyrimidines of which the RNA bases C and U are members. CA is also structurally related to the pyrimidines. Earlier research had shown that these molecules spontaneously formed ring-like structures called rosettes — when immerse in an organic solvent medium. Successive rosettes would stick together to form stacks — compound structures that bear some resemblance to how genetic material is built up from simpler, repetitive structures.
The paper entitled ‘Efficient Self-Assembly in Water of Long Noncovalent Polymers by Nucleobase Analogues’ was published February 8th in the Journal of the American Chemical Society.
For further reading on the subject of biogenesis, check out my earlier article In Search of Ancient Alien Microbes & the Origin of Life.
Main Reference for this post: ‘Self-Assembling Molecules Offer New Clues on Life’s Possible Origin’ by Robert F. Service
Top Image: (RNA-like molecules self-assemble into gene-like structures in water); Credit: B.J. Cafferty et al., JACS (2013)