Spontaneous sister chromatid exchanges detected by a BUdR-labelling method

Abstract

THERE is still controversy over the nature of sister chromatid exchanges detectable by autoradiography. Based on the relation between the number of exchanges and dose of tritium incorporated into DNA, Gibson and Prescott¹ have concluded that all exchanges are radiation-induced and that none occurs spontaneously. But such an approach has an apparent technical drawback in that the declining of sensitivity of emulsion to tritium as well as latent image fading during long autoradiographic exposure provides a definite lower limit in the number of grains formed at a given tritium dose, and hampers greatly the detection of exchanges at very low dose of incorporated tritium. Another difficulty may arise from the fact that the exchange frequency reaches a saturation level at a very low dose and within a very narrow dose range, so that it is difficult to determine whether a dose response curve rises linearly with or as the square of dose. Extrapolation of the curve through the zero point can be misleading. To detect sister chromatid exchanges, I have, following the original work of Latt³, used a new method involving labelling of chromosomes with BUdR followed by acridine orange fluorescence staining, and have obtained results that suggest strongly the existence of spontaneous sister chromatid exchanges.