Hypercycles

Cooperating hypercycles based on multilevel supramolecular structures could behave in an extremely complex way when subject to variable fluxes of energy and matter. No wonder, then, that a single photon produced by the prey hidden in the dark and absorbed by the retinal in the lynx s eye may trigger an enormous 

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Eigen, M. and Schuster, P. (1979), The Hypercycle. A Principle of Natural Self Organization, Springer, Berlin. 

A hypercycle is a more complex organisation form. Its precondition is the presence of several RNA quasi-species which are able to amalgamate chemically with certain proteins (enzymes or their precursors). If such a protein is linked to a quasi-species, the resulting duo favours the replication of a second quasispecies. According to Dyson, the linked populations get stuck in a stable equilibrium. Problems occur at this level Any theory on the origin of replication has the central problem that the replication process must occur perfectly in order to ensure survival . If there are replication errors, these will increase from generation to generation, until the system collapses the error catastrophe has then occurred ... 

This dilemma could be overcome by the hypercycle model hypercycles are in fact not theoretical concepts, but can be observed (in a simple form) in today s organisms, where an RNA virus transfers the information for an enzyme in the host cell, which is able to carry out the preferred synthesis of new virus RNA. This RNA synthesis is supported by host factors, and an RNA minus-strand is formed. The following RNA replication affords a plus-strand. The process corresponds to a double feedback loop and involves the enzyme coded by the RNA matrix and the information present in the matrix in the form of a nucleotide sequence. Both factors contribute to the replication of the matrix, so that there is second-order autocatalysis (Eigen et al., 1982). 

Fig. 8.4 Hypercycle phenomena can be observed when a cell is infected by an RNA virus. The vims provides the host cell with information for an enzyme favouring only the reproduction of viral information, i.e., of an RNA strand. This RNA is converted by the host cell into a protein (a replicase) which forms a new RNA minus-strand. The latter is then replicated to give a plus-strand (Eigen et al., 1982)...

The hypercycle models developed later by Eigen were much more complex. Since both protein enzymes and nucleic acids contribute to hypercycles, the latter could only have come into operation at a later stage of the (hypothetical) RNA world. It seems possible that the protein enzymes on the primeval Earth could have been replaced by ribozymes. 

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 short circuit this happens when an RNA molecule in the hypercycle is changed so much by mutation that it does not catalyse the next reaction in the chain, but a later one. The hypercycle is then short-circuited to become a simple cycle. 

Computer simulations showed that the first two of these catastrophes become more probable as the size of the molecular population increases. In order to avoid them, the population of a hypercycle would need to be kept as small as possible. The probability of collapse, however, decreases with increasing population. Because of these contradictions, Ursula Niesert gave one of her articles the title The Origin of Life between Scylla and Charybdis , because computer simulations indicate that there is only a small interval of hypercycle populations in which all three of the above catastrophes can be excluded. 

As expected, a response to the hypercycle criticisms appeared, in fact in the same issue of the Journal of Theoretical Biology (Eigen et al., 1980). According to this, the Freiburg investigations refer to one particular evolution model, in which the occurrence of mutants with different, selective values is ignored. In such realistic models, the error threshold loses its importance for the stability of the wild type. If the latter reaches a finite fitness value, it can always be the subject of selection, as no rivals are present. 

Genuine self-organisation, i.e., self-organisation as a property of the system. Here, a system with a high degree of complexity organises itself under certain conditions. A typical example is Eigen s hypercycle model (see Sect. 8.3). 

At the point where amphiphiles were recruited to provide the precursors to cell membranes, stable lipid vesicles could have evolved [141] to enclose autocatalytic chiral hypercycles. Credible models for the subsequent evolution of vesicles containing self-replicating chiral molecules have appeared in the literature. [193,194] These vesicles could then emerge from the feldspar spaces [134,192] as micron-sized self-reproducing, energy-metabolizing vesicular systems protobacteria ready to face the hydrothermal world on their own terms. 

Eigen, M. Schuster, P. The Hypercycle Springer-Verlag Berlin, 1979. 

Indeed, water is the medium in which life is believed to have emerged and evolved. This was made possible by the evolution of hydrophobic membranes that allow formation of compartments in which optimal concentrations and gradients of physiological compounds can be maintained and biochemical hypercycles could emerge. Imagine life appearing on a planet of methane seas - it would most likely be supported by hydrophilic membranes. 

BIFURCATION THEORY PRION PLAQUE FORMATION Autocatalytic processes during evolution, HYPERCYCLE AUTOINHIBITION ACTIVATION... 

HYPERCYCLE BIOMASS BIOMIMETIC BIOMINERALIZATION ACCRETIQN IQNIC STRENGTH SQLUBILITY PRQDUCT LANGMUIR ISQTHERM Bio-Rad chromatography columns,... 

Figure 7.12 A simple rendering of an hypercycle. Each of the units A, B, C and D is a replicator. The rate of replication of each unit is an increasing function of the concentration of the unit immediately proceeding it. Thus the rate of replication of B is an increasing function of the concentration of A, and so on round the cycle. (Adapted from Maynard-Smith and Szathmary, 1995.)...

The Hypercycle a Principle of Natural Self-Organization. Springer Verlag. 

Lee, D. H., Severin, K., Yokobayashi, Y, and Ghadiri, M. R. (1997). Emergence of symbiosis in peptide self-replication through a hypercyclic network. Nature, 390, 591. ... 

Triple helix formation expands the catalytic repertoire of template induced oligopeptide formation beyond simple autocatalysis since reaction networks of the hypercycle type (Eigen8 and Schuster, 1979) can be designed with this reaction too. 

Eigen M, Schuster P. The Hypercycle. New York Springer Verlag, 1997-... 

Fig. 8 The hypercycle (a) and its parasite (b). Each member I, is autocatalytic for its own growth and heterocatalytic for the replication of the next member. The parasite P shown accepts the catalytic help from h but does not give anything back. If the arrow leading to P is stronger than that leading to I3, the system is doomed to extinction in a spatially homogeneous dynamical system...

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