Illustration of the basic principle of the Comet assay. When subjected to a strongly alkaline buffer, DNA tends not only to unwind but also to separate the two chains, then exposing single- and double-strand breaks, as well as eventually some alkali-labile sites (e.g. altered nucleotides) that may break under the alkali electrophoresis. Compared to the “neutral” version, the alkaline Comet assay yields not only more fragments but also lower molecular weight fragments, whose speed of migration is therefore higher, thus producing longer “tails”.