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How does it work?

The comet assay is based upon the movement of nuclear DNA through an agarose gel when an electrical field is applied. The theory is that undamaged DNA retains a highly organised association with matrix proteins in the nucleus and when the DNA is damaged (by chemicals or UV radiation), this organisation is disrupted.

During the application of an electric field, the individual strands of DNA lose their compact structure and drift out of the nucleus and into the low melting point agarose suspension. The DNA (which has an overall negative charge) it is drawn towards the positively charged anode. The undamaged DNA does not move, whereas the damaged DNA (smaller fragments) are free to migrate away from the nucleus.

Simply put, the amount of DNA which leaves the nucleus is a measure of the amount of DNA damage to the cell. The brighter and longer the DNA tail, the higher the level of damage. The name 'comet assay' is derived from the appearance of this damaged and undamaged DNA which resembles a comet.