Читать книгу Replicating And Repairing The Genome: From Basic Mechanisms To Modern Genetic Technologies - Kenneth N Kreuzer - Страница 29

DNA replication in both prokaryotic and eukaryotic cells is significantly more complex than in bacteriophage T7, and yet the basic functions and enzymatic mechanisms are very similar. In addition, the structural architecture of many of the key replication proteins, such as DNA polymerases and helicases, are analogous and sometimes evolutionarily related. The distinctions evident in cellular systems include the use of many more protein subunits in the overall replication reaction. One such difference of great importance is the use of specialized proteins, called sliding clamps, which tether cellular replicative polymerases to their DNA template and orchestrate polymerase behavior.

The most extensively studied cellular replication system is that of the bacterium, Escherichia coli, and this remarkably efficient system will be the focus of this chapter. Under optimal conditions, E. coli can duplicate its 4.6-million base-pair chromosome with high accuracy at a speed that allows the cells to divide every 20 minutes. Many bacterial species have replication proteins that are homologous to those of E. coli and presumably replicate by similar mechanisms, with only minor variations. However, it is worth noting that more distantly related bacteria sometimes show significant deviations from the E. coli model. For example, certain grampositive bacterial species such as Bacillus subtilis have an extra replicative polymerase with distinct properties. Without going into details, one of the Bacillus subtilis replicative polymerases is thought to extend the RNA primer on the lagging strand for a short distance, and then hand off the reaction to the other replicative polymerase (which also replicates the leading strand). As we will see in the next chapter, this kind of handoff is also a key feature of eukaryotic DNA replication.