Site-specific segregation

Re combinational DNA repair of a circular bacterial chromosome, while essential, sometimes generates deleterious byproducts. The resolution of a Holliday junction at a replication fork by a nuclease such as RuvC, followed by completion of replication, can give rise to one of two products the usual two monomeric chromosomes or a contiguous dimeric chromosome (Fig. 25-41). In the latter case, the covalently linked chromosomes cannot be segregated to daughter cells at cell division and the dividing cells become stuck. A specialized site-specific recombination system in E. coli, the XerCD system, converts the dimeric chromosomes to monomeric chromosomes so that cell division can proceed. The reaction is a site-specific deletion reaction (Fig. 25-39b). This is another example of the close coordination between DNA recombination processes and other aspects of DNA metabolism. 

FIGURE 25-41 DNA deletion to undo a deleterious effect of re-combinational DNA repair. The resolution of a Holliday intermediate during recombinational DNA repair (if cut at the points indicated by red arrows) can generate a contiguous dimeric chromosome. A specialized site-specific recombinase in E. coli, XerCD, converts the dimer to monomers, allowing chromosome segregation and cell division to proceed. 

The composition of an alloy surface is often very different than the alloy s bulk composition due to segregation effects. The overall activity of a catalyst is determined by the distribution of active sites. This distribution may be very heterogeneous both in terms of the local environments that define each site and their chemical reactivities. The reactivity of any specific active site can be affected by contributions from strain, ligand and ensemble effects. Computational methods are well suited to exploring these effects because one can simulate model systems where only one effect dominates as well as model systems where multiple effects are important. 

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