Self-replication, catalytic

In a self-reproducing, catalytic hypercycle (second order, because of its double function of protein and RNA synthesis) the polynucleotides Ni contained not only the information necessary for their own autocatalytic self-replication but also that required for the synthesis of the proteins Ei. The hypercycle is closed only when the last enzyme in the cycle catalyses the formation of the first polynucleotide. Hypercycles can be described mathematically by a system of non-linear differential equations. In spite of all its scientific elegance and general acceptance (with certain limitations), the hypercycle does not seem to be relevant for the question of the origin of life, since there is no answer to the question how did the first hypercycle emerge in the first place (Lahav, 1999). 

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 point is also made [134] that the very high surface areas and the richly interconnected three-dimensional networks of these micron-sized spaces, coupled with periods of desiccation, could together have produced microenvironments rich in cat-alytically produced complex chemicals and possibly membrane-endosed vesides of bacterial size. These processes would provide the proximate concatenation of lipid vesicular precursors with the complex chemicals that would ultimately produce the autocatalytic and self-replicating chiral systems. A 2.5 km2 granite reef is estimated to contain possibly 1018 catalytic microreactors, open by diffusion to the dynamic reservoir of organic molecules. .. but protected from the dispersive effects of flow and convection [134] as well as protected from the high flux of ultraviolet radiation impinging on the early Earth. [123,137]... 

Shreion Lifson (1997) utilized a nice arithmetic to illustrate the power of the autocatalytic self-replication. He took the example of a normal hetero-catalytic process that makes one molecule of B from A at the rate of one per second. Then, it would require 6 x 10 s to make one mole of B. If instead there is an autocatalytic process by which B gives rises to 2B, and 2B give rise to 4B, and 4B to 8B, and so on, it requires only 79 s to make one mole of B. 

One limit to this beautiful chemistry hes in the requirement of selfcomplementarity of the self-replicating sequences. The more general case of a template working by complementarity was also investigated by von Kiedrowski s group (Sievers et al, 1994). As noted by Burmeister (1998), the underlying principle in this case is a cross-catalytic reaction in which one strand acts as a catalyst for the formation of the other strand. This is illustrated in Figure 7.6. 

Figure 7.6 Minimal representation of a cross-catalytic self-replicating system. (Adapted from Burmeister, 1998.)...

A self-replicating system based on the catalytic action of reversed micelles has been presented in Chapter 1. Other cases of micellar catalysis have been discussed in Ref. 119. The use of semi crystalline fibers to immobilize catalysts and substrates was also proposed [120]. Another approach to the enhancement... 

Selective replication of self-duplicating catalytic RNA segments... 

A self-replicating polymer would quickly use up available supplies of precursors provided by the relatively slow processes of prebiotic chemistry. Thus, from an early stage in evolution, metabolic pathways would be required to generate precursors efficiently, with the synthesis of precursors presumably catalyzed by ri-bozymes. The extant ribozymes found in nature have a limited repertoire of catalytic functions, and of the ribozymes that may once have existed, no trace is left. To explore the RNA world hypothesis more deeply, we need to know whether RNA has the potential to catalyze the many different reactions needed in a primitive system of metabolic pathways. 

Molecular parasites may also have originated in an RNA world. With the appearance of the first inefficient self-replicators, transposition could have been a potentially important alternative to replication as a strategy for successful reproduction and survival. Early parasitic RNAs would simply hop into a self-replicating molecule via catalyzed transesterification, then passively undergo replication. Natural selection would have driven transposition to become site-specific, targeting sequences that did not interfere with the catalytic activities of the... 

Munson (171) carried out experimental work on HCN in zeolites in parallel with the acetonitrile experiments. The results were consistent with oligomerization. Although this process by itself is less interesting from a catalytic standpoint, the oligomerization of HCN and its reaction with formaldehyde remain of interest in the study of prebiotic syntheses of monomers that may have combined to form the first self-replicating systems (172). 

In self-replicating systems employing three starting constituents competition between constituents can occur [9.205]. Such processes are on the way to systems displaying information transfer, whereas the two-components ones are non-infor-mational. A shift from parabolic kinetics to exponential growth of the template concentration is required for a selection process to take place [9.197]. The evidence for self-replication on the basis of template-directed autocatalysis as in 184 requires detailed mechanistic investigation on the origin of the catalytic effects observed [9.206]. 

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. 

This discussion. .. has, in a sense, focused on a straw man the myth of a self-replicating RNA molecule that arose de novo from a soup of random polynucleotides. Not only is such a notion unrealistic in light of our current understanding of prebiotic chemistry, but it should strain the credulity of even an optimist s view of RNA s catalytic potential...Without evolution it appears unlikely that a self-replicating ribozyme could arise, but without some form of self-replication there is no way to conduct an evolutionary search for the first, primitive self-replicating ribozyme. 

The central component of this RNA World scenario is a replication system that can make copies of the genetic material to grow and produce progeny. The most basic scenario is a simple templated ligation of small oligomers in a cyclical replication scheme (13-16), but ultimately a catalytic entity responsible for this crucial function of replication would be necessary. The simplest example is a single self-replicating ribozyme that can copy itself. More elaborate scenarios for... 

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