G A T Tcg Act C

Figure 8.6 Mismatch Repair The endonuclease excises a piece of DNA containing the misincorporated nucleotide. A new complementary strand is then synthesized and joined to the original strand by DNA ligase.

8.4 Repair of Damaged DNA 197

several mechanisms to combat the harmful effects of these rays which result in thymine dimer formation and distortion of the DNA.

In one mechanism, an enzyme can break the covalent bond of thymine dimers, but only in the presence of visible light. The enzyme uses the light energy to cleave the covalent bond. This mechanism is called light repair or photoreactiva-tion and restores the DNA to its original state (figure 8.7a).

Some bacteria also have an enzyme that excises the damaged segment from a single strand of DNA and another enzyme that repairs the resulting break by synthesizing a strand complementary to the undamaged strand. The repaired strand is then joined to the one end of the undamaged region by DNA ligase (figure 8.7b). This process is termed excision repair and corrects damage that causes a distortion in DNA. Because visible light is not required for the action of these enzymes, such repair is also called dark repair.

This same repair mechanism is also important to the well-being of humans. People exposed to the sun, with its UV light, for long periods have a higher incidence of skin cancer than people who are not exposed. Also, people who have a defective dark repair enzyme have an increased incidence of skin cancer. This illustrates both the damage UV light can cause to DNA and the importance of repair mechanisms in overcoming the damage.

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