DNA damage that involves particularly "bulky" molecules or chemical bonds between bases, or that significantly distorts the double-stranded structure of DNA, is subject to repair by the nucleotide excision repair (NER) pathway. For example, it has long been known that the ultraviolet (UV) light in sunshine can damage DNA by forming what are called photoproducts. UV radiation excites many types of molecules, causing them to react with each other and with DNA. In particular, UV light can catalyze the formation of chemical bonds between adjacent thymine and/or cytosine bases; these bonds are called intra-strand UV crosslinks (Figure 4A). These crosslinked bases distort the double-stranded structure of DNA and block DNA replication.
A second example of bulky DNA damage is that caused by large, organic molecules like aflatoxin, found in mold-contaminated peanuts, and benzo[^]pyrene (Figure 4B), a main component of smoke and soot. Both the link between two nucleotides in DNA or RNA
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