LARGE-SCALE FRAGMENTATION OF MAMMALIAN DNA IN THE COURSE OF APOPTOSISPROCEEDS VIA EXCISION OF CHROMOSOMAL DNA LOOPS AND THEIR OLIGOMERS

It has been shown recently that apoptotic degradation of genomic DNA i n mammalian cells starts by excision of large DNA fragments ranging in size from 50 kilobases to more then 300 kilobases. Although it was su ggested that the above fragments could represent chromosomal DNA loops , the supposition was not supported by direct experimental evidence. I n present work, we have studied the specificity of nucleolar and euchr omatic gene long-range fragmentation in mouse and human cells triggere d to undergo apoptosis either by tumor necrosis factor or by serum dep rivation. Separation of the excised large DNA fragments by pulsed fiel d gel electrophoresis followed by Southern analysis has demonstrated t hat in all cases studied the above fragmentation proceeds in a specifi c way. Furthermore, the patterns of DNA long-range fragmentation in th e cells undergoing apoptosis were indistinguishable from the patterns of DNA cleavage into chromosomal loops by the high salt-insoluble topo isomerase II of the nuclear matrix. These results suggest the conclusi on that apoptotic degradation of chromosomal DNA starts by excision of DNA loops and their oligomers.