458Chapter 14 •Mutation, Repair, and Recombinationthe bromine atom signifcantly alters the distribution oFelectrons in the base ring; so 5-BU can frequentlychangeto either the enol Form or an ionized Form; the enol andionized Forms oF 5-BU pair with guanine (±igure 14-7b).5-BU causes G · C:A · T or A · T:G · C transi-tions in the course oF replication, depending on whether5-BU has been enolized or ionized within the DNAmolecule or as an incoming base. Hence the action oF 5-BU as a mutagen is due to the Fact that the moleculespends more oF its time in the enol or ion Form.Another widely employed mutagen is 2-amino-purine (2-AP), an analog oF adenine that can pair withthymine (±igure 14-8a). When protonated, 2-AP can mispair with cytosine (±igure 14-8b). ThereFore, when 2-AP is incorporated into DNA by pairing withthymine, it can generate A · T:G · C transitions bymispairing with cytosine in subsequent replications. Or,iF 2-AP is incorporated by mispairing with cytosine, thenG·C:
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