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Exposure to either ionizing or nonionizing radiation can each induce mutations in DNA, although by different mechanisms. Strong ionizing radiation like X-rays and gamma rays can cause single- and double-stranded breaks in the DNA backbone through the formation of hydroxyl radicals on radiation exposure ( [link] ). Ionizing radiation can also modify bases; for example, the deamination of cytosine to uracil, analogous to the action of nitrous acid. K.R. Tindall et al. “Changes in DNA Base Sequence Induced by Gamma-Ray Mutagenesis of Lambda Phage and Prophage.” Genetics 118 no. 4 (1988):551–560. Ionizing radiation exposure is used to kill microbes to sterilize medical devices and foods, because of its dramatic nonspecific effect in damaging DNA, proteins, and other cellular components (see Using Physical Methods to Control Microorganisms ).
Nonionizing radiation, like ultraviolet light, is not energetic enough to initiate these types of chemical changes. However, nonionizing radiation can induce dimer formation between two adjacent pyrimidine bases, commonly two thymines, within a nucleotide strand. During thymine dimer formation, the two adjacent thymines become covalently linked and, if left unrepaired, both DNA replication and transcription are stalled at this point. DNA polymerase may proceed and replicate the dimer incorrectly, potentially leading to frameshift or point mutations.
| A Summary of Mutagenic Agents | |||
|---|---|---|---|
| Mutagenic Agents | Mode of Action | Effect on DNA | Resulting Type of Mutation |
| Nucleoside analogs | |||
| 2-aminopurine | Is inserted in place of A but base pairs with C | Converts AT to GC base pair | Point |
| 5-bromouracil | Is inserted in place of T but base pairs with G | Converts AT to GC base pair | Point |
| Nucleotide-modifying agent | |||
| Nitrous oxide | Deaminates C to U | Converts GC to AT base pair | Point |
| Intercalating agents | |||
| Acridine orange, ethidium bromide, polycyclic aromatic hydrocarbons | Distorts double helix, creates unusual spacing between nucleotides | Introduces small deletions and insertions | Frameshift |
| Ionizing radiation | |||
| X-rays, γ-rays | Forms hydroxyl radicals | Causes single- and double-strand DNA breaks | Repair mechanisms may introduce mutations |
| X-rays, γ-rays | Modifies bases (e.g., deaminating C to U) | Converts GC to AT base pair | Point |
| Nonionizing radiation | |||
| Ultraviolet | Forms pyrimidine (usually thymine) dimers | Causes DNA replication errors | Frameshift or point |
The process of DNA replication is highly accurate, but mistakes can occur spontaneously or be induced by mutagens. Uncorrected mistakes can lead to serious consequences for the phenotype. Cells have developed several repair mechanisms to minimize the number of mutations that persist.
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