High-Fidelity DNA Excision-Repair Systems Recognize and Repair Damage

In addition to proofreading, cells have other systems for preventing mutations due to copying errors and exposure to chemicals and radiation. Several DNA excision-repair systems that normally operate with a high degree of accuracy have been well studied. These systems were first elucidated through a combination of genetic and biochemical studies in E. coli. Homologs of the key bacterial proteins exist in eukaryotes from yeasts to humans, indicating that these error-free mechanisms arose early in evolution to protect DNA integrity. Each of these systems functions in a similar manner: a segment of the damaged DNA strand is excised, and the gap is filled by DNA polymerase and ligase using the complementary DNA strand as a template.

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We will now take a closer look at some of the mechanisms of DNA repair, ranging from repair of single-base mutations to repair of DNA broken across both strands. Some of these mechanisms accomplish their repairs with great accuracy; others are less precise.