DNA repair mechanism. Illustration of the repair of DNA (deoxyribonucleic acid, red) using excision repair enzymes (blue). At upper left, an excision repair enzyme is moving along a strand of DNA towards a damaged region (black, thymine dimer). The enzyme stops at the damaged area (centre left) and changes into a form that expands to either side (lower left, yellow). This expanded form cuts the DNA (upper right), and a helicase enzyme then unwinds the DNA to be discarded (centre right). Another enzyme (DNA polymerase) then repairs the break with new DNA (blue, lower right). For this artwork without labels, see C023/8899.