Figure 5.3. Spontaneous reactions corrupt the DNA database.

repair enzyme uracil-DNA glycosidase (Fig. 5.3). This leaves a gap in the DNA where the base had been attached to deoxyribose. There is no enzyme that can simply reattach a C into the vacant space on the sugar. Instead, an enzyme called AP endonuclease recognizes the gap and removes the sugar by breaking the phosphodiester links on either side (Fig. 5.5). When DNA has been damaged by the loss of a purine (Fig. 5.3), AP endonuclease also removes the sugar that has lost its base. The AP in the enzyme's name means apyrimidinic (without a pyrimidine) or apurinic (without a purine).

The repair process for reinserting a purine, or a pyrimidine, into DNA is now the same (Fig. 5.5). DNA polymerase I replaces the appropriate deoxyribonucleotide into

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