Final answer:
Double-strand breaks in DNA can be caused by ionizing radiation from radioactive decay or cosmic rays, endogenous damage from metabolic byproducts, and exposure to ultraviolet radiation from the sun.
Step-by-step explanation:
Double-strand breaks (DSBs) in DNA can be caused by a number of insults, including ionizing radiation, which may stem from radioactive decay or cosmic rays. These intense forms of radiation can create hydroxyl radicals upon exposure, resulting in breaks within the DNA strands. Ionizing radiation not only causes single and double-strand breaks but can also alter the bases, such as the deamination of cytosine to uracil.
Another source of DNA damage is endogenous damage arising from reactive oxygen species produced by normal metabolic byproducts, leading to spontaneous mutations—especially oxidative deamination. Lastly, environmental factors like exposure to ultraviolet (UV) radiation can cause damage such as crosslinking between adjacent nucleotide bases, creating pyrimidine dimers, which is a form of direct DNA damage.