Cellular repair enzymes remove virtually all DNA damage before it is fixed; repair therefore
plays a crucial role in preventing cancer. Repair studied at the level of transcription
correlates poorly with enzyme activity, and so assays of phenotype are needed. In a
biochemical approach, substrate nucleoids containing specific DNA lesions are incubated
with cell extract; repair enzymes in the extract induce breaks at damage sites; and the
breaks are measured with the comet assay. The nature of the substrate lesions defines the …