Mutagen

Radiation refers to two different phenomena: light and high-energy particles. Visible light represents a small slice of the electromagnetic spectrum, which includes long-wavelength (low-energy) radio waves and short-wave-length (high-energy) ultraviolet waves, plus X rays and gamma rays. These high-energy forms can directly disrupt DNA by breaking its chemical bonds. In severe cases, this can break apart chromosomes, causing chromosome aberrations. More often, they create mutagenic free radicals in the cell. X rays were first used by Hermann Muller to induce mutations in fruit flies. They continue to be used to create mutations in model organisms to study genes and their effects.

Ultraviolet light is less energetic than X rays but causes mutations nonetheless. The higher-energy form, UV-B, is more toxic than UV-A, because of its potential to cause cross-linking between adjacent thymine or cytosine bases, creating a so-called pyrimidine dimer (cytosine and thymine are chemically classified as pyrimidines). Pyrimidine dimers interrupt replication. UV-A does not cause dimer formation but can still cause mutations by creating free radicals.

Another meaning of the term "radiation" is high-energy particles released during the breakdown of radioactive elements, such as uranium. These particles are either electrons (called beta particles) or helium nuclei (called alpha particles). Their energy is sufficient to disrupt DNA structure, or to create free radicals.