© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.

Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.

4.3 Natural Biological Replicators

Biology has given rise to many different forms of self-replication, from simple to complex. The logical similarities between living organisms and von Neumann’s kinematic automaton have been noted by Sugita [314] and others, and there is clearly also the possibility of engineering artificial versions of these natural systems, especially artificial cellular replicators (Section 4.4). Investigations of artificial molecular replicators might also lead to new insights into the origin of life on Earth [1723, 1724]. For example, Dyson [2397] has suggested that Earth life might have originated by a symbiotic combination of two very different kinds of replicators – non-Mendelian replicating oil droplets enclosing homeostatic metabolic chemical systems that absorbed raw materials from their surroundings, and initially-parasitic molecules of DNA that infected the droplets and took advantage of the existing metabolic machinery in order to reproduce. Compartmentalization is generally regarded as useful for replicative machinery [1725]. Woese [1726] notes that key steps in the origin of life – called crossing the “Darwinian Threshold” – include the incorporation of information storage and feedback mechanisms in supramolecular prebiotic assemblies.

Natural biological replicators include pure protein forms (prions), pure nucleotide forms (viroids), protein-encased nucleotide forms (viruses), and more complex cellular forms (prokaryotes and eukaryotes). Besides simple unit replicators, biology also exemplifies factory-like modes of replication in which two or more separate entities cooperate symbiotically to achieve replication of the whole. In some cases, such as the mitochondrion (Section 4.3.7), the symbiosis has become obligate for all parties – the mitochondrion alone cannot replicate itself (via growth and fission) outside of the cell because it requires the assistance of cell-supplied proteins and lipids, and the cell alone cannot replicate itself because it needs the mitochondria to produce ATP, the principle intracellular energy molecule. Even in viruses where the relationship with infected hosts is usually obligate only for one party (the virus) and is usually considered parasitic, examples of mutually obligate virion-eukaryote symbiotes are known [1727]. In other cases, the symbiosis may be merely facultative. For example, lichens [1728-1731] are two- or three-way symbionts of fungi (the mycobiont), algae (the photobiont), and sometimes also cyanobacteria (a photobiont and nitrogen source) combined into a single body (the thallus). Each of the three components is an independently viable replicator, but by combining together the system can inhabit a wider variety of environments. The several hundred Chlorella algae which are dispersed throughout the cytoplasm of Paramecium bursaria are another example of facultative symbiosis among independent cellular replicators [1732].

Last updated on 1 August 2005