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P.4 caption to figure 2: "An overview of types of DNA damage and causal agents (Helleday, Eshtad and Nik-Zainal 2014). Sources of DNA damage include endogenous factors such as spontaneous or enzymatic conversions. The N-glycosidic bond that links a nucleobase and a pentose sugar to form a nucleoside is labile. This fact underlies the common occurrence of spontaneous base loss in DNA (∼10,000 bases per cell per day), which results in the formation of apurinic or apyrimidinic sites. Other types of endogenous DNA damage include deamination, polymerase errors and free radical species which can induce the formation of DSBs [Double-strand breaks]. By contrast, ultraviolet radiation is responsible for the formation of pyrimidine dimers, which can be mutagenic when left unrepaired. Other external agents that are known to cause DNA damage include chemical compounds which can cause bulky adducts or interstrand and intrastrand crosslinks, intercalating agents, DNA alkylating agents and psoralens." |