DNA damage, due to environmental factors and normal metabolic processes inside the cell, occurs at a
rate of 1,000 to 1,000,000 molecular lesions per cell per day. While this counts for only a small part of
the human genome's approximately 6 billion bases (3 billion base pairs), unrepaired lesions to critical
genes can impede a cell's ability to carry out its function and appreciably increase the likelihood of
cancer.
The comet assay, or single cell gel electrophoresis assay (SCGE), is a common technique for
measurement of DNA damage in individual cells. Under an electrophoretic field, damaged cellular DNA
(containing fragments and strand breaks) is separated from intact DNA, yielding a classic “comet tail”
shape under the microscope. Extent of DNA damage is usually visually estimated by comet tail
measurement; however, image analysis software is also available for measuring various parameters.