Final answer:
The statement that depurination of DNA is rare and caused by ultraviolet irradiation is false. UV radiation typically causes pyrimidine dimers, not depurination, which can occur spontaneously and is more common in certain extreme environments like those found in thermophilic bacteria.
Step-by-step explanation:
The statement “Depurination of DNA is a rare event” is false. Depurination refers to the hydrolytic removal of purine bases (adenine or guanine) from the DNA molecule. This process can occur spontaneously and is not directly caused by ultraviolet irradiation. Instead, UV radiation typically causes pyrimidine dimers, which is a bonding of adjacent pyrimidines like thymine or cytosine. Depurination occurs more frequently at elevated temperatures, such as those found in thermophilic bacteria. These organisms experience a high depurination rate which they may be adapted to handle beyond normal repair mechanisms.
Furthermore, UV exposure can lead to other types of DNA damage such as oxidative damage, inappropriate methylation, and DNA strand breakage. However, depurination is not among the primary effects of UV light on DNA. The earth's ozone layer filters out the most harmful UV radiation, but lower-energy UV-B radiation can reach the surface and cause DNA damage. Repair enzymes like photolyase can reverse certain types of damage in organisms that possess this enzyme, although humans lack it.