RNA-world theory

One of the problems with DNA is that it is essentially a linear code that stores information. The functional groups (nucleotides) only interact with each other and, while this can lead to the elegant double helix, it limits the ability of DNA to form different secondary and extensive tertiary structures. RNA is rather more amenable to forming other structural motifs, hence the RNA World theory of molecular evolution, but it appears that only proteins with their varied side chains are able to adopt truly complex structures. 

Despite the difficulties outlined above, the intellectual elegance of the RNA world theory of the origin of life32 has led many scientists to seek abiotic routes to the components of RNA. That can be illustrated with the chemical instability of ribose. 

RNA copies of the genome would have directly acted as the chemical workhorses of this primitive life. This concept is known as the RNA world theory. 

The considerable problems associated with the RNA world led to a search for simpler systems. Nelson et al. (2000a, b) sum up the greatest problems of the theory as follows ... 

The RNA world requires a system capable of self-replication as a precondition for the beginnings of life. In contrast, the surface metabolism theory proposed by Wachtershauser postulates that the initial step is metabolism, from which complex replication systems can evolve later. This metabolism would have occurred at the... 

The theory that life began in an RNA World suggests that the first self-replicating system was a set of RNA molecules. The catalytic and informationtransferring properties of RNA indicate a possible scenario ... 

The ribosome is where the message carried by the mRNA is translated into the amino sequence of a protein. How it occurs is described in the next section. One of its most noteworthy aspects was discovered only recently. It was formerly believed that the RNA part of the ribosome was a structural component and the protein part was the catalyst for protein biosynthesis. Present thinking tilts toward reversing these two functions by ascribing the structural role to the protein and the catalytic one to rRNA. RNAs that catalyze biological processes are called ribozymes. Catalysis by RNA is an important element in origins of life theories as outlined in the accompanying boxed essay RNA World. ... 

Among the hot debate between theories concerning either a gradual change from RNA self-replication to RNA coded protein synthesis as in the RNA-world, or a merger of systems involving co-evolved RNA and catalytic peptides, another type of molecule - lipids - have until recently been overlooked. 

The origin-of-life scenario was instrumental for the new theory of the cell, because it led to the the conclusion that the ribotype had an evolutionary priority over genotype and phenotype. More precisely, the scenario described a precellular ribotype world (not to be confused with the RNA world) where some ribosoids could act as templates (ribogenotype), others as enzymes (ribophenotype), and others as polymerising ribosoids (ribotype) that were responsible for the growth and the quasi-replication of the ribonucleoprotein systems. 

An RNA that exhibits enzyme-Hke activity is called a ribozyme. The discovery of ribozymes had a great impact on research into the origins of life. Identifying catalytic capabilities in RNA, an information molecule, led to a new theory the RNA world hypothesis. This suggests that RNA was the first life form on Earth, and when it first evolved it performed both catalytic and enzymatic functions. The natural selection process associated with evolution eventually caused the RNA to evolve into the highly sophisticated supramolecular systems observed in the complex life forms present today. 

In the early 1990s C. de Duve suggested the sulfur-based theory of life origin. The author has described the thioester world as a precursor of an RNA world (Figure 25). 

We can share the view of some authors (Fenchel et al., 1998) that this theory may serve a plausible explanation of thioester world as precursor for the present RNA world. Furthermore, it is attractive in a number of respects too. One has to mention the development of very early, global scale geochemical cycling of carbon, sulfur, and iron. It would present the early interactions between atmosphere and biochemical processes, like thioester-bounded hydrogen production contribution to the formation of reducing species that were critical for any atmospheric organic synthesis. 

We expect that we will soon see examples of ribo- and deoxyribozymes evolved for the catalysis of complex chemical transformations. There is enough reason to assume that such synthetic enzymes will be used as catalysts in organic syntheses. The novel catalysts not only support theories of an RNA world ,... 

Ellington AD Experimental testing of theories of an Early RNA World. Molecular Evolution Producing the Biochemical Data 1993, 224 646—664. 

The same group also demonstrated that an enzymatic RNA replication can occur within self-replicating liposomes, however, with no dependency between the content and the container during the replication process. As for the previous examples, liposomes were loaded with all the necessary reagents and their self-reproduction occurred using oleic anhydride at 40 °C. This example represents an autopoietic process as the reproduction of the boundary is due to the reaction that takes place within the boundary and is catalytically induced by the boundary itself. This work is presented as the first example in which the reproduction of the membrane and the replication of the internalized RNA molecules proceed simultaneously and thus, the first bridge established between the two more accepted views on the theory of minimal life the RNA world and the cellular autopoietic view. It should be mentioned that in a former... 

---