Dna damage can occur as a result of exposure to chemicals or ultraviolet radiation. This is what happens during nucleotide excision repair of damaged dna. A multienzyme complex crosses the DNA in search of distortions in the double helix, once located, the phosphate skeleton of the affected chain is cut on both sides of the altered region, a helicase eliminates the oligonucleotide resulting from digestion, the "Gap" "is filled with a DNA polymerase and a ligase.