The comet assay is a versatile and sensitive method for measuring single- and double-strand breaks in DNA. The mechanism of formation of comets (under neutral or alkaline conditions) is best understood by analogy with nucleoids, in which relaxation of DNA supercoiling in a structural loop of DNA by a single DNA break releases that loop to extend into a halo—or, in the case of the comet assay, to be pulled towards the anode under the electrophoretic field. A consideration of the simple physics underlying electrophoresis leads to a better understanding of the assay. The sensitivity of the assay is only fully appreciated when it is calibrated: between one hundred and several thousand breaks per cell can be determined. By including lesion-specific enzymes in the assay, its range and sensitivity are greatly increased, but it is important to bear in mind that their specificity is not absolute. Different approaches to quantitation of the comet assay are discussed. Arguments are presented against trying to apply the comet assay to the study of apoptosis. Finally, some of the advantages and disadvantages of using the comet assay on lymphocyte samples collected in human studies are rehearsed.

Engineered nanoparticles (NPs) are widely used in different technologies but their unique properties might also cause adverse health effects. In reviewing recent in vitro and in vivo genotoxicity studies we discuss potential mechanisms of genotoxicity induced by NPs. Various factors that may influence genotoxic response, including physico-chemical properties and experimental conditions, are highlighted. From 4346 articles on NP toxicity, 112 describe genotoxicity studies (94 in vitro, 22 in vivo). The most used assays are the comet assay (58 in vitro, 9 in vivo), the micronucleus assay (31 in vitro, 14 in vivo), the chromosome aberrations test (10 in vitro, 1 in vivo) and the bacterial reverse mutation assay (13 studies). We describe advantages and potential problems with different methods and suggest the need for appropriate methodologies to be used for investigation of genotoxic effects of NPs, in vitro and in vivo.