Fig. 3. DNA repair incorporation of ddNTPs catalyzed by whole-cell extracts. (A) Reaction mixtures (10 µl) containing 10 ng of substrate oligonucleotide duplex with normal (lanes 1–3) or reduced (lanes 4–6) AP sites were incubated with 10 µg of human cell extracts for 20 min at 37°C without dNTPs (lanes 1 and 4) in the presence of normal dNTPs (lanes 2 and 5) or with ddNTPs added (lanes 3 and 6). (B) Reaction mixtures (10 µl) containing 10 ng of substrate oligo nucleotide duplex with reduced AP sites were incubated with 10 ng of APE1 (lane 1) or 10 µg of Pol β-deficient cell extracts (lanes 2–4) for 20 min at 37°C without dNTPs (lane 2) or in the presence of ddNTPs (lanes 3 and 4). The reaction shown in lane 4 also contains 5 ng of purified Pol β. Reaction products were analyzed by electrophoresis in 20% denaturing polyacrylamide gel.