Indirect DNA damage results from the interaction with which species?

• A. A free radical
• B. A free electron
• C. An x-ray photon
• D. Ion pairs

Answer: (A)

Indirect DNA damage happens when radiation interacts with water in the cell and splits it into reactive chemical species. The most important of these are free radicals, especially the hydroxyl radical. These radicals diffuse briefly and chemically attack DNA, causing damage such as strand breaks or base changes without the radiation needing to hit the DNA directly. That’s why the best answer is a free radical—the reactive species generated from water radiolysis that drives indirect damage.

Free electrons, x-ray photons, and ion pairs play roles in the initial energy deposition, but they are not the primary agents causing indirect DNA damage. Ion pairs form first, and free electrons can be produced, but the damaging interactions with DNA in the indirect pathway come from the radicals produced when those ions react with water and other molecules.