MoBio Mutation by UV light Chapter 7

UV light may cause two adjacent pyrimidine residues (cytosine or thymine) to form a dimer. In a normal cell, the dimer can be detected by p53, which then triggers the repairing process. However, if p53 itself is mutated and become non-functional, the pyrimidine dimer may lead to mutation.

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Figure 7-F-4. Pyrimidine dimer induced by UV light. This figure uses thymine as an example. Cytosine may form a similar dimer.

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Figure 7-F-5. A possible mechanism for the mutation induced by UV light.

In the above figure, the UV light first causes two adjacent cytosine residues to form a dimer. During DNA replication, both strands are used as templates to synthesize new strands. The cytosine dimer could cause adenine (instead of the normal guanine) to be incorporated into the new strand. Subsequent DNA replication will produce CC to TT mutation. Although the cytosine dimer may eventually be corrected, the mutation cannot be detected by the DNA repair system.