460Chapter 14 •Mutation, Repair, and Recombinationpolymerase (DNA polymerase III) stalls at a site of DNAdamage, the DNA ahead of the polymerase continues tobe unwound, exposing regions of single-stranded DNAthat become bound by single-strand-binding protein (SSB).Next, RecA proteins join the SSB and form a protein–DNA Flament. The RecA Flament is the biologically active form of this protein. In this situation, RecA acts asa signal that leads to the induction of the error-pronepolymerase and attracts it to the stalled fork.Mutagens that create bases unable to form stablebase pairs are thus dependent on SOS and similar sys-tems for their mutagenic action, because the incorpora-tion of incorrect nucleotides requires the activation ofthe SOS system. The category of SOS-dependent mu-tagens is important because it includes most cancer-causing agents (carcinogens), such as ultraviolet (UV)light and a±atoxin B1. Indeed, a great deal of work hasbeen done on the relationship of mutagens to carcino-gens. The connections between mutation and cancer will
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.