Evidence for the Internucleosomal Breakage of Chromatin in Rat Thymocytes Irradiated in Vitro

Low molecular-weight (free) DNA increases in lymphatic cells in association with radiation-induced interphase death. In this report, the size of the free DNA from rat thymocytes irradiated in vitro with 1-kR x rays was analyzed by agarose gel electrophoresis and compared with that of the DNA fragments extracted from micrococcal nuclease-digested thymocyte nuclei and with that of HaeIII-digested phi X-174 RF DNA. Results demonstrated clearly that the free DNA consisted of a series of discretely sized DNA fragments with lengths that were integral multiples of a nucleosomal DNA (180 base pairs), suggesting that internucleosomal chromatin breaks occurred in rat thymocytes after in vitro irradiation. Additional evidence was obtained from an electron microscopic observation on the breakdown of chromatin strands into particle form in the irradiated cells.