O radicals and guanine

Harvard Gazette: Repairing DNA damage: "During these searches, OGG1 finds millions of normal guanines for every aberrant one. It must be able to detect very subtle differences between good and bad, and the molecular snapshots taken by Verdine's group reveal how it does that.
OGG1 boasts a tiny pocket in its structure that acts as a trap. Normal guanine won't fit into the pocket. Mutated guanine, carrying an extra oxygen atom, fits smoothly.
That oxygen atom can cause problems. It may come from radiation received for treatment of cancer, breathing polluted air, or normal breakdown of food we eat. In the last case, the oxygen is usually converted to water, releasing life-giving energy in the process. But occasionally, a highly reactive form of oxygen escapes into the body, as can also happen when someone breathes polluted air or receives treatment with radiation.
Called 'free radicals,' these oxygen atoms react ferociously with anything nearby. Most of the molecules they attack are constantly being replenished, so no harm is done. But if they react with a base like guanine, problems occur. "