O o

GROWTH

GROWTH

FILTRATION
Fig. 5.6. Growth and division oflipid vesicles. (See text for details.)

and ionic strength. The process by which the vesicles formed from micellar solutions required several hours, apparently with a rate limiting step related to the assembly of 'nuclei' of bilayer structures. However, if a mineral surface in the form of clay particles was present, the surface in some way catalysed vesicle formation, reducing the time required from hours to a few minutes. The clay particles were spontaneously encapsulated in the vesicles. The authors further found that RNA bound to the clay was encapsulated as well. In a second series of experiments, Hanczyc and coworkers (Hanczyc et al., 2003; Hanczyc and Szostak, 2004) showed that the myristoleic acid vesicles could be induced to grow by addition of fatty acid to the medium, presumably by incorporating fatty acid molecules into the membrane, rather than by fusion of vesicles. If the resulting suspension of large vesicles was then filtered through a polycarbonate filter having pores 0.2 ^m in diameter, the larger vesicles underwent a kind of shear-induced division to produce smaller vesicles which could undergo further growth cycles (Figure 5.6). This remarkable series of experiments clearly demonstrated the relative simplicity by which complex system of lipid, genetic material and mineral catalysts can produce a model protocellular structure that can undergo a form of growth and division.

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