Vesicle self-reproduction

So far, only the self-replication mechanisms of linear molecules have been described it is now time to consider closed spherical structures, such as micelles and vesicles. Here, the term self-reproduction will be used rather than self-replication, because, as it will be seen, the population increase is generally based on statistical processes. The subject of micelles and vesicles self-reproduction is dealt with in other chapters in this book a certain degree of repetition and/or mis-match is unavoidable. 

Vesicle self-reproduction described until now can be defined as autopoietic, since growth and eventually reproduction comes from within the structure boundary. One can also induce growth and division of fatty acid vesicles by adding fresh surfactant from the outside, for example as a micellar solution at high alkaline pH. 

Bozic, B. and Svetina, S. A relationship between membrane properties forms the basis of a selectivity mechanism for vesicle self-reproduction. European Biophysics Journal, 33 (2004), 565-71. 

Type I system. Self-reproduction of vesicles. The starting point is the concept of vesicle self-reproduction. This important vesicle pattern was observed for the first time in 1994 and thoroughly studied till recently, being one of the most important reactive behaviour of fatty acid vesicles. Figure 17.8a shows the experimental model for autopoietic... 

P. Stano, E. WehrU, P. L. Luisi, Insights on the oleate vesicles self-reproduction, J. Phys. Condens. Matter, 2006, 18, S2231AS2238. 

Figure 7.16 Self-reproduction scheme of vesicles S-S represents a water-insoluble precursor that binds to the bilayer and is hydrolyzed in situ.

In general, the mechanism of self-reproduction of micelles and vesicles can be considered an autopoietic mechanism, since growth and eventually division comes from within the structure itself. This point will be considered again in Chapter 8, on autopoiesis, where the mechanism of the self-reproduction process will also be discussed. 

In the various reviews on self-reproduction in recent years, practically no mention is made of such micelles or vesicle systems. The reason lies most prohahly in the bias of classic hiochemical literature, according to which self-replication is tantamount to nucleic acid systems lacking this are therefore deemed not to he relevant. In this particular regard, it is argued that self-reproduction of micelles and vesicles proceeds without transmission of information. 

We have also learned that self-replication is not a prerogative only of nucleic acids, but it can be shared by different kinds of chemical families see the formose reaction, the self-replicating peptides, and the self-reproducing micelles and vesicles. The list should include the cellular automata and the corresponding devices of artificial life. Self-reproduction of vesicles and liposomes is important because it represents a model for cell reproduction. 

The term chemical autopoiesis indicates the experimental implementation of autopoiesis in the chemistry laboratory. The most well known of these processes is the self-reproduction of micelles and vesicles. This has been discussed in the previous chapter, where the original idea of Francisco Varela and myself was to work with bounded systems that would produce their own components due to an internal reaction, respecting the scheme illustrated in Figure 8.3. We came up with the idea of using reverse micelles (refer back to Figure 7.13) with two reagents. 

When the velocities of the two reactions are numerically equal, the system is in homeostasis, a dynamic equilibrium that does not modify the identity of the unit. Conversely, when the velocity of the building-up reaction Vp is larger than the opposite one, growth and eventually self-reproduction of the vesicles can be measured and if instead Vp is smaller than uj, there is destruction of the unit. 

Reverse micelles are the first compartment structures for which the phenomenon of micelle self-reproduction has been described (Bachman et al., 1990 1991). This experimental work was a follow up of a theoretical study by Varela and Luisi (Luisi and Varela, 1990), and is it this that eventually brought to light the self-reproduction of aqueous micelles and vesicles. This has been covered already in Chapter 7, on the chemistry of self-reproduction. 

More convincing evidence has been obtained for the synthesis of simpler compounds, such as straight-chain fatty acids. This observation is important because, as we have already seen in the chapter on self-organization and self-reproduction, these compounds form stable vesicles. Prebiotic synthesis of these compounds was reported for example by Nooner et al. (1976). More recently monocarboxylic acids have been observed from a spark discharge synthesis (Yuen et al, 1981) and from a Fischer-Tropf type of reaction (McCollom et al, 1999 Rushdi and Simoneit,... 

Vesicles are commonly considered models for biological cells. This is due to the bilayer spherical structure which is also present in most biological cells, and to the fact that vesicles can incorporate biopolymers and host biological reactions. Self-reproduction, an autocatalytic reaction already illustrated in the chapters on self-reproduction and autopoiesis, also belongs to the field of reactivity of vesicles. Some additional aspects of this process will be considered here, together with some particular properties of the growth of vesicles - the so-called matrix effect. 

In the case of Walde et al. (1994a) the synthesis of poly(A) - which can be viewed as a simple form of RNA - proceeded simultaneously with the self-reproduction of vesicles, thus providing a core and shell reproduction, as schematically illustrated in Figure 10.3. 

It has already been discussed in Chapters 7 (self-reproduction) and 8 (autopoiesis) that, under certain conditions, vesicles are capable of undergoing an autocatalytic process of self-reproduction. This is a novel, dynamic aspect of the reactivity of such aggregates, which clearly has relevance for the field of the origin of life. 

In the meantime, the intense study of the simpler vesicle systems has unravelled novel, unsuspected physicochemical aspects - for example growth, fusion and fission, the matrix effect, self-reproduction, the effect of osmotic pressure, competition, encapsulation of enzymes, and complex biochemical reactions, as will be seen in the next chapter. Of course the fact that vesicles are viewed under the perspective of biological cell models renders these findings of great interest. In particular, one tends immediately to ask the question, whether and to what extent they might be relevant for the origin of life and the development of the early cells. In fact, the basic studies outlined in this chapter can be seen as the prelude to the use of vesicles as cell models, an aspect that we will considered in more detail in the next chapter. 

Vesicles can self-reproduce, however it is argued that there is no information content passing from one generation to the next. What do you think is information really so important for self-reproduction of early protocells ... 

This is illustrated in Figure 11.3. It consists of a vesicle containing two ribozymes, one (Rib-2) capable of catalyzing the synthesis of the membrane component the other (Ribl) being an RNA replicase that is capable of repUcating itself, and reproducing the Rib-2 as well. In this way, there is a concerted shell-and-core replication, and there is therefore a basic metabolism, self-reproduction, and - since the replication mechanism is based on RNA replication - also evolvability. 

Polynucleotides phosphorilase producing poly(A) from ADP 2. poly(A) is produced inside simultaneously with the (uncoupled) self-reproduction of vesicles. 

Self-reproduction of micelles and vesicles models for the mechanisms of life from the perspective of compartmented chemistry. Adv. Chem. Phys., 92,425-38. 

Morigaki, K., Dallavalle, S., Walde, P, Colonna, S., and Luisi, P. L. (1997). Autopoietic self-reproduction of chiral fatty acid vesicles. /. Am. Chem. Soc., 119, 292-301. 

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