Ultraviolet light generates a number of distinct types of alterations in DNA, called photoproducts, from the word photo for “light.” The most likely to lead to muta-tions are two different lesions that unite adjacent pyrim-idines in the same strand. These lesions are the cyclobu-tane pyrimidine photodimer and the 6-4 photoproduct. For the cyclobutane pyrimidine dimer, ultraviolet light stimulates the formation of a four-membered cyclobutyl ring (shown in green in Figure 14-12a) between two ad-jacent pyrimidines on the same DNA strand by acting on 5,6 double bonds. The 6-4 photoproduct structure (Figure 14-12b) forms between the C-6 and C-4 posi-tions of two adjacent pyrimidines, most prevalently 5 9-CC-3 9 and 5 9-TC-3 9 . The UV photoproducts signi±-cantly perturb the local structure of the double helix. These lesions interfere with normal base pairing; hence, induction of the SOS system is required for mutagenesis. The incorrect bases are inserted across from UV photo-
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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.