Darwin's Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design

by Stephen C. Meyer

As an example of regulated mutation, Shapiro observes that in response to environmental assault—UV damage from sunlight or the presence of an antibiotic, for instance—bacteria activate what is known as the “SOS response” system. This system makes use of specialized error-prone DNA polymerases, normally left unexpressed, that are synthesized and set into action, allowing the population to generate a much wider range of genetic variation than usual. Bacterial cells regulate this process using a DNA-binding protein known as LexA, which normally represses the error-prone polymerases. When the SOS system is activated by environmental damage, the production of LexA first drops dramatically, allowing expression of the error-prone polymerases, but then rises, which “ensures that as soon as DNA repair occurs . . . LexA [will] reaccumulate and repress the SOS genes.”59 This system allows cells to “replicate DNA that carries unrepaired damage,”60 keeping their essential replication machinery moving past a stall, in the absence of which the bacterium would die.