Recombination sexual

Figure 11.11 shows examples of the three basic genetic operations of reproduction, crossover and mutation, as applied to a population of 8-bit chromosomes. Reproduction makes a set of identical copies of a given chromosome, where the number of copies depends on the chromosome s fitness (see below). The crossover operator exchanges subparts of two chromosomes, where the position of the crossover is randomly selected, and is thus a crude facsimile of biological sexual recombination between two single-chromosome organisms. The mutation operator randomly flips one or more bits in the chromosome, where the bit positions are randomly chosen. 

The evolutionary history of symbiotic nitrogen fixers is therefore a tale of coevolution, which occurred in the shadow of their hosts, chasing their growing roots, and striving for adaptation. It is an example of how bacterial genetics has managed to keep pace with the creative power of eukaryotic sexual recombination. Mobile replicons, insertion elements, and symbiotic islands prone to move have helped rhizobia to succeed in their pursuit. The race, naturally, is not over and, looking at it from a distance, what we have. seen, compared to what we have yet to see, is probably just a cloud of dust. 

The realization that one could screen or select beneficial point mutations or recombinations and discard deleterious ones, thus mimicking sexual recombination, spawned the field of directed evolution. 

A breakthrough was achieved by recognizing that the process of natural selection can be harnessed to evolve effective enzymes in artificial circumstances. In directed evolution the processes of natural evolution are accelerated in a test tube in order to select proteins with the desired properties. The realization that one could screen or select beneficial point mutations or recombinations and discard deleterious ones, thus mimicking sexual recombination, spawned the field of directed evolution. 

True Darwinian evolution involves multiple cycles of mutation and selection. This process can be mimicked in a laboratory setting to optimize the properties of an inefficient enzyme. The hexameric but weakly active chorismate mutase [95] described in Section 3.3.4.1 has been improved in this way [99]. Mutations were introduced into the gene encoding the hexamer subunit by DNA shuffling (Fig. 3.16) [5, 100], which mimics sexual recombination in vitro. Improved variants were selected, as before, by their ability to complement the chorismate mutase deficiency in bacteria. Plasmid DNA was isolated from the fastest growing cells and the entire procedure was repeated. 

Another S. sclerotiorum mutant obtained in this study, SL-1, cannot form sclerotia. Sclerotia are morphological structures that serves both as precursors for fruiting bodies (ascocarps) and for dormant survival during adverse conditions (5). This mutant is unable to make spores, or survive the winter, or undergo sexual recombination, resulting in its demise during the winter months. 

Natural biological evolution occurs as a result of naturally occurring mutations, fragmentation and loss of DNA, duplication, and other genetic processes. Furthermore, sexual recombination increases the possibility of successful genetic combinations. The selection of improved variants occurs as a result of environmental conditions or selection pressures. 

In a landmark paper, Muller (1964) demonstrated that clonal lineages may be subject to reduce fitness due to a relentless increase in the load of deleterious mutations, and that sexual recombination would tend to reduce that load. The progressive increase in deleterious mutations has become known as Muller s ratchet, and there is evidence for the ratchet in nonrecombining genomes present in eukaryotic cells, namely, mitochondrial DNA (Lynch, 1997 Moran, 1996), sex chromosomes, and Drosophila chromosomes that are prevented from recombining (Rice, 1994). A potential consequence of the ratchet is extinction of clonal lineages (Lynch et al., 1993). Some models indicate that clonal species... 

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