The Emergence of Competition Between Model Protocells

Life is that which replicates and evolves. The origin of life is also the origin of evolution. A fundamental question is when do chemical kinetics become evolutionary dynamics? Here, we formulate a general mathematical theory for the origin of evolution. All known life on earth is based on biological polymers, which act as information carriers and catalysts. Therefore, any theory for the origin of life must address the emergence of such a system. We describe prelife as an alphabet of active monomers that form random polymers. Prelife is a generative system that can produce information. Prevolutionary dynamics have selection and mutation, but no replication. Life marches in with the ability of replication: Polymers act as templates for their own reproduction. Prelife is a scaffold that builds life. Yet, there is competition between life and prelife. There is a phase transition: If the effective replication rate exceeds a critical value, then life outcompetes prelife. Replication is not a prerequisite for selection, but instead, there can be selection for replication. Mutation leads to an error threshold between life and prelife.prelife ͉ replication ͉ selection ͉ mutation ͉ mathematical biology T he attempt to understand the origin of life has inspired much experimental and theoretical work over the years (1-10). Many of the basic building blocks of life can be produced by simple chemical reactions (11)(12)(13)(14)(15). RNA molecules can both store genetic information and act as enzymes (16)(17)(18)(19)(20)(21)(22)(23)(24). Fatty acids can self-assemble into vesicles that undergo spontaneous growth and division (25-28). The defining feature of biological systems is evolution. Biological organisms are products of evolutionary processes and capable of undergoing further evolution. Evolution needs a generative system that can produce unlimited information. Evolution needs populations of information carriers. Evolution needs mutation and selection. Normally, one thinks of these properties as being derivative of replication, but here, we formulate a generative chemistry (''prelife'') that is capable of selection and mutation before replication. We call the resulting process ''prevolutionary dynamics.'' Replication marks the transition from prevolutionary to evolutionary dynamics, from prelife to life.Let us consider a prebiotic chemistry that produces activated monomers denoted by 0* and 1*. These chemicals can either become deactivated into 0 and 1 or attach to the end of binary strings. We assume, for simplicity, that all sequences grow in one direction. Thus, the following chemical reactions are possible:[1]Here i stands for any binary string (including the null element). These copolymerization reactions (29, 30) define a tree with infinitely many lineages. Each sequence is produced by a particular lineage that contains all of its precursors. In this way, we can define a prebiotic chemistry that can produce any binary string and thereby generate, in principle, unlimited information and diversity. We call such a sys...

mentioning

confidence: 99%

Prevolutionary dynamics and the origin of evolution

Nowak

Ohtsuki

2008

“…The emergence of any genetically encoded function that enhanced protocell reproduction or survival would have marked the beginnings of Darwinian evolution and, thus, of biology itself. We have been attempting to test various aspects of this model for the origin of cellular life through the laboratory construction and characterization of model protocells (2,3). A necessary and complementary aspect of this program is the exploration of environmental scenarios in which external inputs of matter and energy could drive the replication of the genetic material of a protocell (4), as well as protocell membrane growth and division (5,6).…”

mentioning

confidence: 99%

Photochemically driven redox chemistry induces protocell membrane pearling and division

Zhu

Adamala

Zhang

et al. 2012

Self Cite

108

101

Prior to the evolution of complex biochemical machinery, the growth and division of simple primitive cells (protocells) must have been driven by environmental factors. We have previously demonstrated two pathways for fatty acid vesicle growth in which initially spherical vesicles grow into long filamentous vesicles; division is then mediated by fluid shear forces. Here we describe a different pathway for division that is independent of external mechanical forces. We show that the illumination of filamentous fatty acid vesicles containing either a fluorescent dye in the encapsulated aqueous phase, or hydroxypyrene in the membrane, rapidly induces pearling and subsequent division in the presence of thiols. The mechanism of this photochemically driven pathway most likely involves the generation of reactive oxygen species, which oxidize thiols to disulfide-containing compounds that associate with fatty acid membranes, inducing a change in surface tension and causing pearling and subsequent division. This vesicle division pathway provides an alternative route for the emergence of early self-replicating cell-like structures, particularly in thiol-rich surface environments where UV-absorbing polycyclic aromatic hydrocarbons (PAHs) could have facilitated protocell division. The subsequent evolution of cellular metabolic processes controlling the thiol:disulfide redox state would have enabled autonomous cellular control of the timing of cell division, a major step in the origin of cellular life.origin of life | vesicles O ne model for the nature of the earliest and simplest cells, or protocells, is that they consisted of a self-replicating membrane compartment (vesicle) that encapsulated a self-replicating genetic polymer (1). The emergence of any genetically encoded function that enhanced protocell reproduction or survival would have marked the beginnings of Darwinian evolution and, thus, of biology itself. We have been attempting to test various aspects of this model for the origin of cellular life through the laboratory construction and characterization of model protocells (2, 3). A necessary and complementary aspect of this program is the exploration of environmental scenarios in which external inputs of matter and energy could drive the replication of the genetic material of a protocell (4), as well as protocell membrane growth and division (5, 6). For example, we recently showed that activated nucleotides, added to a suspension of model protocells, could cross the vesicle membrane and copy encapsulated nucleic acid templates (4). We have also shown that large multilamellar fatty acid vesicles can grow by absorbing fatty acid molecules from added fatty acid micelles (6). This growth pathway begins with the emergence of a thin membranous filament from the side of the initially spherical vesicle; the filament grows and gradually incorporates the contents and membranes of the parent vesicle, which is transformed into a long filamentous vesicle. Under modest shear forces, the filamentous vesicle divides into multiple da...

“…Since then, vesicles encapsulating biologically active compounds have been used as chemical microreactors (2), delivery vehicles for pharmaceuticals (3), and platforms for synthetic biological systems (4,5). Because mixtures of lipids and biomolecules will not spontaneously organize into solution-encapsulating vesicles of defined composition and size, several methods have been devised to form and load vesicles including swelling (6), extrusion (7), electroformation (8), electroinjection (9), and reverse evaporation and emulsion (10,11).…”

mentioning

confidence: 99%

Unilamellar vesicle formation and encapsulation by microfluidic jetting

Stachowiak

Richmond

et al. 2008

262

223

Compartmentalization of biomolecules within lipid membranes is a fundamental requirement of living systems and an essential feature of many pharmaceutical therapies. However, applications of membrane-enclosed solutions of proteins, DNA, and other biologically active compounds have been limited by the difficulty of forming unilamellar vesicles with controlled contents in a repeatable manner. Here, we demonstrate a method for simultaneously creating and loading giant unilamellar vesicles (GUVs) using a pulsed microfluidic jet. Akin to blowing a bubble, the microfluidic jet deforms a planar lipid bilayer into a vesicle that is filled with solution from the jet and separates from the planar bilayer. In contrast with existing techniques, our method rapidly generates multiple monodisperse, unilamellar vesicles containing solutions of unrestricted composition and molecular weight. Using the microfluidic jetting technique, we demonstrate repeatable encapsulation of 500-nm particles into GUVs and show that functional pore proteins can be incorporated into the vesicle membrane to mediate transport. The ability of microfluidic jetting to controllably encapsulate solutions inside of GUVs creates new opportunities for the study and use of compartmentalized biomolecular systems in science, industry, and medicine.vortex ͉ liposome ͉ drug delivery ͉ synthetic biology