The comet assay is a technique to measure DNA damage and other DNA alterations in individual cells. The technique aquired its name from the comet-like shape of the DNA of the cells which can be seen under the microscope after the procedure. The technique is also named single cell gel electrophoresis assay, which is more descriptive. The main steps of the procedure are preparation of a cell suspension with the cells of interest, embedding cells in an agarose gel and spreading them on an object slide, lysis of cells to remove non-DNA compounds, exposing the DNA to a neutral or alkaline buffer, subjecting the gels with the DNA to electrophoresis (which forms the "comets"), and measuring the extent of migration and stretching of DNA by microscopical analysis. As many cells are measured (e.g. 100 cells per object slide), the statistical distribution of the individual cell measurements represents the amount of DNA damage/alteration of the cell sample.

On a molecular basis, the types of DNA alterations which can be measured with this technique, are DNA single and double strand breaks and DNA cleavage in general, but also molecule crosslinks, which lead to less electrophoretic migration instead of more. As different types of events - from toxic influences to cellular processes - can lead to these types of alterations, comet assay results are interpreted to explain DNA damage as well as DNA repair or apoptotis.

The comet assay has advantages compared to some other genotoxicity tests as it does not require proliferating cells, it is relatively sensitive, the procedure is quite fast and simple, and it can be applied to a wide range of different cell types and cell samples.

The development of the comet assay followed some techniques which measured DNA diffusion without applying any electrophoresis at all (”Nucleoid Sedimentation”: Cook et al. 1976; ”Halo-Assay”: Roti Roti & Wright 1987). Like in these methods, the first version of the comet assay (with electrophoresis) was carried out in a neutral buffer (Rydberg & Johanson 1978, Östling & Johanson 1984). This version mainly detects DNA double strand breaks. The alkaline version, which additionally detects DNA single strand breaks, was firstly applied by Singh et al. (1988).

• Cook,PR, Brazell,IA, Jost,E (1976): Characterization of nuclear structures containing superhelical DNA. J. Cell Sci. 22, 303-324.
• Östling,O, Johanson,KJ (1984): Microelectrophoretic study of radiation-induced DNA damages in individual mammalian cells. Biochemical and Biophysical Research Communications 123, 291-298.
• Roti Roti,JL, Wright,WD (1987): Visualization of DNA loops in nucleoids from HeLa cells: assays for DNA damage and repair. Cytometry 8, 461-467.
• Rydberg,B, Johanson,KJ (1978): Estimation of DNA strand breaks in single mammalian cells. In: DNA Repair Mechanisms. (Eds: Hanawalt,PC, Friedberg,EC, Fox,CF) Academic Press, New York, 465-468.
• Singh,NP, McCoy,MT, Tice,RR, Schneider,EL (1988): A simple technique for quantitation of low levels of DNA damage in individual cells. Exp. Cell Res. 175, 184-191.