the breakage of one phosphoester bond, creating a nick in the backbone of the helix. The breakage can be either 3' or 5' of the site, depending on the variant of the enzyme. End processing enzymes then prepare the site for nick ligation, which is performed by DNA polymerase. The base inserted into the nick is determined by the corresponding base on the opposite strand. The nick is then sealed by DNA ligase. The activity of AP endonuclease in the repair of AP sites in the frontal/parietal cortex, cerebellum, brain stem, midbrain and hypothalamus declines with age in rats on an ad libitum diet. In calorie restricted rats, by comparison, AP endonuclease activity in these brain regions remains higher with age. These findings suggest that elevated AP site repair in calorie restricted animals may delay the aging process. References Molecular genetics DNA repair