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Figure 4 Methods of repairing damage to DNA bases. (a) Nucleotide excision repair: 1 - DNA with damaged base, 2 -damaged DNA is unwound and separated, 3 - damaged section is excised, 4 - gap is filled by DNA polymerase, 5 - single-strand gap remains after gap filling, 6 - ligase connects free ends. (b) Base excision repair: 1 - DNA with damaged base, 2 - damaged base is removed and nick is made in DNA, 3 - DNA polymerase simultaneously displaces damaged section (producing a 'flap') and synthesizes new DNA, 4 - flap is cut, leaving a single-strand nick, 5 - DNA ligase connects the two free ends of the nick.

and is illustrated in Figure 4b. Another type of DNA repair is DSB repair, which may involve homologous recombination or direct end rejoining. In homologous recombination (Figure 5), a damaged DNA strand is repaired using its homolog as a template (e.g., the maternal copy of a chromosome is used as a template if the paternal copy is damaged and vice versa). This process involves removal of damaged nucleotides and synthesis of new DNA. Homologous recombination may also be used to repair some DNA-DNA cross-links and some SSBs. Other cross-links are repaired in a process that combines aspects of homologous recombination and nucleotide excision repair. In end joining, the damaged bases at the ends of the break are removed and the broken ends are directly joined. Because no template is used to ensure

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