Radiation is energy traveling through space that can interact with biological tissues. A mutation is a permanent alteration in the genetic code, or deoxyribonucleic acid (DNA), of a cell. High-energy radiation can transfer enough energy to cause structural changes in the DNA molecule, meaning some types of radiation induce mutations in living organisms.
How Radiation Damages DNA
Ionizing radiation carries enough energy to knock electrons out of atoms, creating ions and highly reactive molecules. Examples include X-rays, gamma rays, and alpha particles. Non-ionizing radiation generally does not possess the energy required to cause mutations directly.
Ionizing radiation primarily damages DNA through two processes: direct action and indirect action. Direct action occurs when a radiation particle directly strikes the DNA molecule. This collision breaks chemical bonds within the DNA backbone, often resulting in single-strand or double-strand breaks.
Indirect action is the more common method of genetic damage because cells are largely composed of water. The radiation particle hits a water molecule, leading to radiolysis. This process generates highly unstable species, such as hydroxyl free radicals. These free radicals then chemically attack nearby DNA, causing base damage or strand breaks.
The Body’s Mechanisms for DNA Repair
Cells have evolved systems to maintain genetic integrity. Following damage, the cell initiates a DNA damage response that includes repair pathways. These enzyme systems recognize lesions, like strand breaks or damaged bases, and excise the faulty section before synthesizing a correct replacement.
For more serious damage, such as a double-strand break, the cell employs complex mechanisms like non-homologous end joining or homologous recombination to reconnect the severed DNA ends. The success of these repair systems determines if the damage is fixed or if it becomes a permanent mutation.
If the genetic damage is too extensive or incorrectly repaired, the cell can activate apoptosis. This mechanism eliminates the severely damaged cell before it can divide and propagate the mutation. A mutation only occurs when the damage overwhelms the cell’s repair capacity or when the repair process itself introduces an error.
Variables That Determine Mutation Risk
The total absorbed dose (the amount of energy deposited in the tissue) is a primary factor; a greater dose leads to a higher probability of unrepaired damage.
The rate at which the dose is delivered, known as the dose rate, plays a large role. A low dose delivered over a long period allows cellular repair systems time to fix damage as it occurs, reducing the overall mutational risk. Conversely, the same total dose delivered rapidly is more likely to overwhelm the repair machinery.
The type of radiation is also important. High linear energy transfer (LET) radiation, like alpha particles, deposits its energy densely, causing complex, clustered breaks that are harder for the cell to repair than the scattered damage from low-LET radiation like X-rays. Rapidly dividing cells, such as those in bone marrow or the intestinal lining, are more sensitive to radiation-induced damage than mature, non-dividing cells. Damage is more likely to become permanent during cell replication, making proliferating tissues more susceptible to mutation.
Health Outcomes of Unrepaired Mutations
Unrepaired DNA damage can lead to two major categories of health outcomes. The first involves somatic mutations in body cells. If the mutation occurs in a gene that controls cell growth, it can start carcinogenesis.
Unrepaired damage can activate oncogenes or inactivate tumor suppressor genes, leading to the uncontrolled cell division that characterizes cancer. For radiation-induced cancer, a latent period of many years often passes between the initial mutagenic event and the clinical appearance of the disease. Leukemia is a recognized outcome due to the high sensitivity of blood-forming cells.
The second category involves germline mutations. Since these changes occur in reproductive cells and contribute to the genetic makeup of offspring, a mutation can be passed down to the next generation. While radiation causes genetic changes, these effects have been difficult to conclusively detect in human studies.