Cross-catalytic self-replication

Figure 7.6 Minimal representation of a cross-catalytic self-replicating system. (Adapted from Burmeister, 1998.)...

We still need to clear up one or two points of nomenclature in normal replication of nucleic acids, the matrix (the + strand) and the newly formed daughter strand (- strand) are held together by Watson-Crick hydrogen bonding. This process is also referred to as cross-catalytic . Normal autocatalysis is different it leads to a product which corresponds in structure to the matrix, so that there is no difference between the + and - strands. Such self-complementary sequences are called palindromes. 

One limit to this beautiful chemistry hes in the requirement of selfcomplementarity of the self-replicating sequences. The more general case of a template working by complementarity was also investigated by von Kiedrowski s group (Sievers et al, 1994). As noted by Burmeister (1998), the underlying principle in this case is a cross-catalytic reaction in which one strand acts as a catalyst for the formation of the other strand. This is illustrated in Figure 7.6. 

Self-replication can occur through autocatalysis with a self-complementary template (Fig. 2A) or by cross-catalysis with a complementary template (Fig. 2B). In the latter case, two complementary units catalyze each other s formation. This is significant as modern DNA is made up of two complementary strands that replicate in a cross-catalytic manner. Studies on a simple nonenzyinatic DNA system capable of competing auto- and crosscatalysis. show that in the absence of an initial template, both pathways are equally as likely, but that in the presence of a seeding template, the cross-catalytic route is more efficient. 

The model of self-replication discussed thus far involves self-complementary structures. However, template effects in a replication cycle can also operate in a reciprocal sense. In a minimal system, the template is self-complementary, whereas in a reciprocal system, a pair of templates are complementary to each other. Therefore, reciprocal replicating systems rely on two interlinked cross-catalytic cycles in which the two templates catalyze the formation of each other. The processes involved in reciprocal replication are encapsulated schematically in Figure 1(b). 

In this context, an ingenious modification of the original pair of von Kiedrowski s nucleotides led to a network of interdependent auto- and cross-catalytic replicators." The four building blocks used for this purpose are depicted in Figure 6. Fragments Ap and nB are identical to the self-replicating system described previously except for the... 

An example of a DNA network with multiple catalytic routes was studied by Achilles and von Kiedrowski. They analyzed a seff-repUcating system starting from three building blocks, which could form a five different member network with six catalytic pathways. Another example was presented by Sievers and von Kiedrowski, who studied a quaternary system of hexadeoxynucleotide analogs. They demonstrated that cross-catalytic replication can be observed in template-directed reaction systems if the reciprocal template effects are similar in efficiency and if the reaction system is not dominated by autocatalytic syntheses of self-complementary products, which occur as parallel reactions. 

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