45914.1 Point mutationshelix (Figure 14-10b). In this intercalated position,an agent can cause single-nucleotide-pair insertions ordeletions.BASE DAMAGEA large number of mutagens damageone or more bases; so no speci±c base pairing is possible.The result is a replication block, because DNA poly-merase cannot continue DNA synthesis past such a dam-aged template base. In both prokaryotes and eukaryotes,such replication blocks can be bypassedby inserting non-speci±c bases. In E. coli, this process requires the activa-tion of the SOS system.SOS and other mechanisms ofbiological repair will be described later in this chapter.However, an overview of this repair mechanism will bepresented in this section because, somewhat ironically,some repair mechanisms are themselves responsible formutating DNA. The name SOScomes from the idea thatthis system is induced as an emergency response to pre-vent cell death in the presence of signi±cant DNA dam-age. As such, SOS induction is a mechanism of last
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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.