Visual quantification of DNA double-strand breaks in bacteria

In this paper, we describe a method for the visualization of double-strand breaks in a single electrostretched Escherichia coli DNA molecule. We also provide evidence that electrostretched or migrated DNA under neutral microg el electrophoresis conditions is made up of individual chromosomes. Using t he neutral microgel electrophoresis technique, DNA migration (stretching) w as measured and the number of DNA double-strand breaks were counted followi ng exposure of E. coli cells to 0, 12.5, 25, 50, or 100 rad of X-rays. The use of an intense fluorescent dye, YOYO and custom-made slides have helped us in visualizing individual bacterial DNA molecules. Bacterial DNA appears similar in structure compared to electrostretched DNA from human lymphocyt es. We were able to detect changes in DNA migration (stretching) induced by an X-ray dose as low as 12.5 rad and an increase in the number of DNA brea ks induced by a dose as low as 25 rad. The extent of DNA migration and numb er of breaks were directly correlated to X-ray dosage. (C) 1999 Elsevier Sc ience B.V. All rights reserved.