How can ultraviolet light and X-rays cause mutations?

Ultraviolet (UV) light and X-rays cause mutations through distinct physical and chemical interactions with DNA.

Ultraviolet Radiation

UV light is mutagenic primarily because it causes the covalent linking of adjacent pyrimidine bases, a process known as pyrimidine dimerization. These dimers distort the DNA structure and prevent accurate replication. Additionally, UV light can induce photosensitization, where energy is transferred to molecular oxygen to create reactive "singlet oxygen." This reactive species can attack and damage various cellular components, including DNA.12

X-rays

As a form of ionizing radiation, X-rays possess enough energy to strip electrons from atoms, creating ions that disrupt molecular structures. X-rays cause mutations by:3
* Breaking DNA strands: They can induce both single-strand and double-strand breaks in the DNA molecule.23
* Chromosomal damage: By breaking chromosome fibers, X-rays can lead to gene deletion or translocation, where broken ends of chromosomes rejoin incorrectly, altering the genetic sequence.4
* Free radical production: X-rays create free radicals within the cell, which can oxidize DNA bases, particularly guanine, leading to replication errors.2