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linking function (e.g. ribozyme)

Ribozyme Replication Protocell

where an RNA replicase and a self-replicating membrane-bound vesicle combine to form a protocell. Inside the vesicle, the RNA replicase functions, and might add a function to improve the production of the vesicle wall through a ribozyme. At this point, the RNA replicase and the vesicle are functioning together, and the protocell has become a living cell, capable of nutrition, growth, reproduction and evolution. Read a general introduction to RNA world at http://www.blackwellpublishing.com/ paleobiology/. (Based on information in Szostak et al. 2001.)

where an RNA replicase and a self-replicating membrane-bound vesicle combine to form a protocell. Inside the vesicle, the RNA replicase functions, and might add a function to improve the production of the vesicle wall through a ribozyme. At this point, the RNA replicase and the vesicle are functioning together, and the protocell has become a living cell, capable of nutrition, growth, reproduction and evolution. Read a general introduction to RNA world at http://www.blackwellpublishing.com/ paleobiology/. (Based on information in Szostak et al. 2001.)

molecule cannot both replicate and trigger that replication. The minimum requirement is that two RNA molecules interact, one to act as the enzyme to bring together the components, and the other to act as the gene/template. Together the template and the enzyme RNA combine as an RNA replicase. But these components have to be kept together inside some form of compartment or cell, otherwise they would only occasionally come into contact to work together. Szostak and colleagues then proposed there must be a second precellular structure they call a self-replicating vesicle, a membrane-bound structure composed mainly of lipids (organic compounds that are not soluble in water, including fats) that self-replicates, or grows and divides from time to time. The RNA replicase at some point entered a self-replicating vesicle, and this allowed the RNA replicase to function efficiently.

This is a protocell, but it is not yet living. It is just a self-replicating membrane bag with an independent self-replicating molecule inside. To make the protocell function as an integrated cell, the RNA replicase has to carry out a function that benefits the membrane component. For example, the RNA replicase might generate lipids for the membrane through the medium of a ribozyme. With the membrane keeping the RNA replicase together and so improving its function, and the RNA replicase producing lipids for the membrane, the protocell has become a cell. The two functions are coupled, and the cell can evolve, as vesicles with improved ribozymes can grow and divide, and become more abundant than others. So, we have life and we have evolution. The cell is alive because it has the ability to feed itself, to grow and to replicate. Evolution can happen because the cells show differential survival ("survival of the fittest"), and the genetic information for replication is coded in the RNA.

A number of researchers have carried out experiments to explore all these steps in the hypothetical RNA world model. They have succeeded in evolving ribozymes capable of a broad class of catalytic reactions, including linking components of RNA and lipid molecules, and over time the molecules are selected to perform more efficiently. Much work has yet to be done to show how the whole process could have worked, especially to improve the efficiency and accuracy of copying from the template. The other aspect of the model is the self-replicating vesicle. Experiments here have focused on simple physical models for how oily droplets might incorporate freefloating lipids, and so grow, and then how the droplets or vesicles might divide when they reach a certain size or when external forces are applied, perhaps by the movement of

waves in the water. The experiments are complex, and investigators are continuing to explore the behavior of simple RNA replicase, self-replicating vesicles and how the two could come to function together (Szostak et al. 2001).

If the RNA world existed, when was this and for how long? The Earth had to be cool enough for the organic elements to survive being burned off, and the RNA world must pre-date any traces of modern forms of life. Some estimate that this might have been a time of 100-400 myr, somewhere between 4.0 and 3.5 Ga.

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