RNA world

The basic question about the origin of life have always been how the early cells are formed. Basically, the modern cells comprise of a plasma membrane surrounding water in which varieties of chemicals are dissolved and subcellular structures are kept afloat. It appears more likely that the first self replicating substance was much simpler then the most primitive modern living cells.

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Before the existence of life, most evidence found implies that Earth was a very different place, torrid and anoxic with an atmosphere rich in carbondioxide, nitrogen and water vapours. In the ocean and seas, geological and chemical processes forms methane, hydrogen and carboxylic acid. The conditions that allows chemical to react with one another, randomly "testing" the usefulness of the reactions and the stability of the product as provided by areas near hydrothermal vents or in shallow pools. Energy are released from some reactions and these eventually became the fundamental basis of modern cellular metabolism. Some other reaction generated utility molecules could function as catalysts, some combine with other molecules to form the precursors of modern cell structures and others were able to replicate and would act as units of hereditary information.

In modern cells, the role of catalyst, structural molecules and hereditary molecules are performed by three (3) different molecules. Protein performs two major roles in the cells: structural and catalytic. Enzymes are known as catalytic protein and they help to alter the manifolds of chemical reaction that occurs in the cell by speeding them up. Hereditary information are stored by DNA and can be replicated to pass the information from one generation to the other. RNA is majorly involved in converting the information kept in DNA into protein. Judging from all these, don't you think any hypothesis about the origin of life must account for the evolution of these molecules? but No, the very nature of their interactions with one another in modern cells complicates the attempts to imagine or find out how they evolved.

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As demonstrated in its function, protein can do cellular work, their synthesis also involves other protein and RNA, also uses the information that are stored in the DNA. Lol. On the other hand, DNA are unable to do cellular work, instead, it serves as storage for genetic information and as overlay for it's own replication; a process that requires protein also. However, RNA is synthesized using DNA as the template and protein as the catalyst for the reaction.

Base on the above consideration, it was hypothesized that at some point in the evolution of life, there would have been a single molecule, that is able to do both cellular work and replicates itself. This leads to the discovery of a possible molecule in 1981, when a catalytic RNA molecule was discovered in a protist (Tetrahymena sp.) By Thomas Cech. This molecule could cut an internal part of itself and connect the remaining part back together. Not so long after then, other catalytic RNA molecules have been found, including an RNA discovered in Ribosomes that is responsible for the formation of peptide bond - the bond that holds amino acid together. Ribozymes is now used as a term for catalytic RNA molecules.

After the discovery of ribozymes, it was suggested that RNA at some point had the ability to catalyze it's own replication, also using itself as a template.

RNA world is a term coined by Walter Gilbert in 1986 to delineate a precellular stage in the evolution of life in which RNA is found to be capable of copying, storing and also expressing genetic information, even going as far as catalyzing other chemical interactions. It seems however that for the precellular stage to proceed to the evolution of cellular life forms, the RNA must have a lipid membrane around it. This significant evolutionary step is easier to imagine than other events that occurs in the origin of cellular life forms due to lipid being the major structural components of modern organisms membranes and liposomes is being fomed spontaneously. In 2003, three scientists: Marin Hanczyc, Jack Szostak and Shelly Fujikawa performed a fascinating experiment that shows that formation of liposomes that actually grow and divide can be triggered by clay. All these facts together with the ribozymes data suggests that early cells may have been RNA molecules that contains liposomes.

Taking aside it's ability to fulfill catalytic activities, other functions of RNA suggests it's ancient origin. Also putting into consideration of the fact that much of the cellular pool of RNA in modern day cells exist in a structure known as Ribosomes. This structure consists majorly of rRNA (Ribosomal RNA) and uses mRNA (messenger RNA) and tRNA (transfer RNA) to construct or synthesize protein. Remember that rRNA itself catalyzes the formation of peptide bond during the synthesis of protein. Thus RNA seems to be well placed for its significance in protein development. Also recall that because RNA and DNA are structurally similar, RNA could have given rise to the double stranded DNA. Once evolved, DNA becomes the storage facility for genetic information because it is able to provide a more chemically stabilized structure. Some other two pieces of evidence supports the RNA world hypothesis: The recent discovery that RNA can carry out the regulation of genetic expression and that ATP, being the energy currency of the cell is a ribonucleotide. So it seems DNA, protein and cellular energy can all be traced back to RNA.

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Summary

It goes without saying that the emergence of this RNA world and the transition to a DNA world imply an impressive number of stages, each more improbable than the previous one — François Jacob, 1997

Through our knowledge from modern day organisms and what they are comprises of, it seems logical that the progression of the catalytic mechanism essential to living cells started with the evolution of families of molecules that have the ability to catalyse their own replication. Over time, a group of RNA catalyst probably cooperate and gained the ability to synthesis polypeptides directly. DNA could have been a late addition as the accumulation of other protein catalyst allowed more complex and efficient cells to evolve, then the RNA is replaced by DNA so as to allow a more stable molecule to be able to store increase amount of genetic information required by such complex cells.

Despite the hypothesis supporting evidences of an RNA world, it is not without controversies and many scientists still argue against it though that will be extensively discussed another time.

Questions and arguments will be entertained in the comment section or through a post as to make the topic interactive and also sharing your views.

References

@Suesa The Building Blocks of Life - Basics

The RNA World
and Other Origin-of-Life Theories

RNA world