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Microbiology, 2021

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11.5 • Mutations 437<br />

Figure 11.23 Bacteria have two mechanisms for repairing thymine dimers. (a) In nucleotide excision repair, an enzyme complex<br />

recognizes the distortion in the DNA complex around the thymine dimer and cuts and removes the damaged DNA strand. The correct<br />

nucleotides are replaced by DNA pol I and the nucleotide strand is sealed by DNA ligase. (b) In photoreactivation, the enzyme photolyase<br />

binds to the thymine dimer and, in the presence of visible light, breaks apart the dimer, restoring the base pairing of the thymines with<br />

complementary adenines on the opposite DNA strand.<br />

CHECK YOUR UNDERSTANDING<br />

• During mismatch repair, how does the enzyme recognize which is the new and which is the old strand?

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