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

Abstract
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 .vphi.X-174 RF DNA (molecular weight marker). 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 EM observation on the breakdown of chromatin strands into particle form in the irradiated cells.