The indirect action of DNA damage results from interaction with which of the following?

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

Answer: (A)

Indirect DNA damage from ionizing radiation mainly comes from reactive species produced when water in the cell is radiolyzed. Radiation splits water molecules to form free radicals, especially the hydroxyl radical, which are highly reactive and diffuse to DNA to cause strand breaks and base damage. This is why free radicals are the best explanation for the indirect action. Free electrons are produced too, but they’re not the primary damaging agents in this indirect pathway; an x-ray photon can cause direct ionization of DNA (direct action) rather than through radicals; ion pairs are intermediates that lead to radicals, but the damaging species that actually injure the DNA are the radicals themselves.