464 Chapter 14 • Mutation, Repair, and Recombination The deamination of cytosine yields uracil (Figure 14-18a). Unless corrected, uracil residues will pair with ade-nine in the course of replication, resulting in the conver-sion of a G · C pair into an A · T pair (a G · C : A·T transition). Deamination of 5-methylcytosine also occurs (Figure 14-18b). (Certain bases in prokaryotes and eu-karyotes are normally methylated.) The deamination of 5-methylcytosine generates thymine (5-methyluracil). Thus, C to T transitions generated by deamination are also seen frequently at 5-methylcytosine sites. DNA sequence analysis of hot spots for G · C : A · T transitions in the lacI gene has shown that 5-methylcytosine residues are present at the position of each hot spot. Some of the data from this lacI study are shown in Figure 14-19. The height of each bar on the graph represents the frequency of mutations at each of a number of sites. The positions of 5-methylcytosine residues correlate nicely with the
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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.