Structural alterations to DNA molecules caused by agents such as ionizing radiation.DNA damage refers to any physical or chemical change in the structure of a DNA molecule that alters its normal information content or blocks its copying and reading. The most common lesions include single- and double-strand breaks in the sugar-phosphate backbone, missing or altered bases, and cross-links that staple the two strands together. Ionizing radiation produces damage in two ways: it can strike the molecule directly, or it can split water molecules nearby into highly reactive radicals that then attack the DNA. A single chest X-ray or a cosmic-ray hit at altitude deposits energy in a cell, and the resulting lesions are the starting point for mutations, cell death, or, if misrepaired, cancer. Because every cell in the body suffers thousands of such events per day from normal metabolism and background radiation, life depends on repair systems that find and fix the damage before it is copied.
The link between radiation and biological damage was recognized in the early 20th century, soon after X-rays and radioactivity were discovered. Hermann Muller's 1927 demonstration that X-rays cause mutations in fruit flies established that radiation damages genetic material. The chemical nature of the lesions—strand breaks, base modifications—was worked out from the 1950s onward as molecular biology tools improved.
When ionizing radiation passes through tissue, it leaves a trail of ionized atoms. Along that trail, water molecules break into hydroxyl radicals, which abstract hydrogen atoms from the DNA bases or attack the backbone. A single radiation track can produce clustered damage: several lesions within a few base pairs, which are harder to repair than isolated ones. Direct hits can also knock electrons out of the DNA itself. The cell responds with repair pathways: base excision repair removes a single damaged base, nucleotide excision repair cuts out a short stretch containing the lesion, and homologous recombination or non-homologous end joining reconnects broken strands. If repair fails or is inaccurate, the damage becomes a permanent mutation.
Radiation therapy for cancer deliberately inflicts DNA damage on tumor cells, while radiation protection standards and dosimetry are based on quantifying the damage risk to healthy tissue. DNA repair inhibitors are also being tested as drugs to make tumors more sensitive to radiation.
A single mammogram delivers a radiation dose equivalent to about three days of natural background radiation, and the resulting DNA damage is usually repaired within hours.