How do ultraviolet light, X rays, and gamma rays cause mutations?
Damage to DNA is caused by photons of ultraviolet light, X rays, and gamma rays (but not by photons of lower energy, such as visible light, microwaves, and radio/TV waves). The highest energy photons (X rays and gamma rays) cause the breakage of covalent bonds, leading to a variety of damaging effects on DNA. Photons of ultraviolet light get absorbed by DNA, causing a rearrangement of covalent bonds to give the production of “pyrimidine dimers” (usually involving two thymines), as shown in Figure 14.27. DNA damage caused by UV light, X rays, or gamma rays may kill a cell unless the damage is repaired before or during the next round of DNA replication. The mutagenic effects of these forms of radiation are primarily due to errors that occur during the repair process; i.e., the repair job is good enough to keep the cell alive but may not return the DNA to exactly the same state. 2. Has “transposition” ever been a significant cause of mutations in humans? Our genome is loaded with “transpo
