Producing protocells

Approaches to Protocells

protocells_fig2

Fig. 2 The bottom-up approach focuses on assembling a minimal protocell from simple inorganic and organic components. The top-down approach focuses on the simplification of modern cells. Eventually these two approaches will meet in the middle. 

One way to produce a protocell is to combine an appropriate mix of nonliving molecules and then let them react and self-assemble into a living protocell. This process begins with molecular constituents and so may be considered a bottom-up approach to protocells. Most of the work in (Rasmussen et al., 2008 and Sole et al., 2007) falls within this bottom-up approach. Complementing this is a top-down approach based on the recent success of genomic research. The top-down approach begins with an existing contemporary living cell, typically a very simple one, and reduces its genome by successive removal of genes, to arrive at a minimal cell with just enough genes to maintain itself and reproduce (Hutchison et al., 1999). One long-range aim of this top-down research is to make an artificial cell by destroying a cell’s original genome and inserting a new minimal genome that is synthesized externally from nucleotides using genomic technology (Glass et al., 2006). A spectrum of intermediate levels of functional organization is spanned by top-down approaches beginning with living matter (contemporary cells) and bottom-up approaches beginning with nonliving matter, as illustrated in figure 2. 


Once a population of protocells exists, their functional effectiveness might cause some to be selected over others. If there is a combinatorially large family of possible forms of informational control, and if information inheritance is neither too perfect nor too imperfect, then the process of evolution might be able to improve those functionalities over time. Evolvability is often thought to be an essential property of life (e.g., Maynard Smith, 1975), but it remains elusive in existing protocell realizations. In fact, it is an open question under which conditions such a system will be able to exhibit open-ended evolution, by which we mean a system’s ability to create new properties in an open-ended manner over time as a result of selection and environmental pressures (Bedau et al., 2000). Statistical analysis of evolutionary systems, including artificial life models, indicates that the only systems known to exhibit open-ended evolution are natural life (the biosphere) and human technology, which is itself generated by living entities (Bedau et al., 1997; Skusa and Bedau, 2002). One key to achieving open-ended evolvability is the ability of evolving systems to generate novel properties. Novel properties in molecular systems can be obtained not only by genetic rearrangement but also by aggregation of existing systems. For example, combining a lipid membrane system with a photosynthesizer might produce a simple light-driven metabolic system. Aggregation-generated novelty could be as important as genetic variation in the evolution of protocells. 


All protocell architectures presented in PACE rely crucially on the process of self-assembly. This means that the resulting protocell objects are not designed or constructed piece by piece, as happens with the products of traditional engineering; rather, the requisite materials are brought together under the appropriate laboratory conditions and the protocells spontaneously form. Protocells are themselves emergent structures, so designing them and controlling their functionality will require the development of new techniques of emergent engineering. 


© 2004-2008 All rights reserved by PACE Consortium .   Email.   Web Managers: U. Tangen & J. S. McCaskill