resynthesis (step 2), and ligation (step 3). In step 1, the damage is cut out by one of a series of nucleases, each specialised for a type of DNA damage; in steps 2 and 3, the original DNA sequence is restored by a repair DNA polymerase, which fills in the gap created by the excision events. DNA ligase seals the nick left in the sugar-phosphate backbone of the repaired strand. Nick-sealing, which requires energy from ATP hydrolysis, remakes the broken phosphodiester bond between the adjacent nucleotides. Some types of DNA damage (deamination of cytosine, for example) involve the replacement of a single nucleotide, as shown in the figure. For the repair of other kinds of DNA damage, such as thymine dimers, a longer stretch of 10 to 20 nucleotides is removed from the damaged strand.
