Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 72

While the process described above and diagrammed in Figure 1.10 would suffice for pristine DNA on a template that lacked any type of physical block to the progression of the DNA replication complex, in reality, the situation in the cell is rarely this tidy. DNA polymerases frequently encounter a number of different problems. Challenges to DNA polymerases include interruptions in the DNA template, bulky adducts that cannot be replicated by DNA polymerase III, and physical blocks mediated by supercoiling and proteins bound to, or acting on, the chromosome. One extreme form of impediment to a replication fork comes from nicks in either the leading-strand or lagging-strand template DNAs in which the phosphatedeoxynucleotide chain is broken in one strand of the DNA. These nicks cause the destruction of the nick-containing arm of the replication fork, resulting in a broken chromosome and collapse of the DNA replication fork. Bacteria possess a highly efficient mechanism for priming repair of this broken end by using the broken DNA itself as a primer to reinitiate DNA replication. This process was likely the original driving force for the evolution of recombination and is described in chapter 9.