Answer to Question #1745 Submitted to "Ask the Experts"

Q

Why is x radiation a more potent mutagen than ultraviolet (UV) radiation?

A

The reason that ionizing radiation (such as x radiation) is a more potent mutagen than UV is highly technical. Ionizing radiation has the ability to break chemical bonds, creating ion pairs (hence its name). Its action is totally random and indiscriminate. Thus, it can directly ionize DNA. Water is the most likely molecular target, since it is the most common molecule in living systems. Water ions rapidly create free radicals, which then attack macromolecules, such as DNA. In general, UV radiation is less energetic than x-rays and while some of the most energetic UV photons are capable of producing ionization, many UV photons are nonionizing. The UV radiation may induce mutagenesis through a molecular mechanism that results in the formation of thymidine dimers. That is, it crosslinks two thymidine residues in DNA. In addition to these differences in production of molecular damage, there are differences in cellular capabilities to repair these damages. Conclusion: Differences in mutagenic effectiveness between ionizing radiation and UV relate to differences in action at the molecular level. S. Julian Gibbs, DDS, PhD