[Thiol-induced fragmentation of chromosomal DNA]

Supramolecular complexes of DNA (SC DNA) were isolated from loach sperm, loach erythrocytes and hen erythrocytes by the phenol method. By the use of UV-sedimentation on neutral 5-20% sucrose gradient, we studied the effect of 2-mercaptoethanol (ME), dithiothreitol (DTT) and NaBH4 on SC DNA at different pH and long-time incubation (5 and 10 days). It appeared that ME treatment at pH 4.4 fragmented SC DNA of three objects into subunits of size 5 x 10(5)D. Incubation with DTT at pH 8 in the presence of EDTA resulted in subunits of size 1-2 x 10(7)D. However, NaBH4 at pH 8 failed to induce fragmentation of SC DNA. It is shown that ME-induced at pH 4.4 fragmentation is accompanied by a decrease in hyperchromatic effect of subunits, indicating the presence of "sticky" ends. Thus, ME-induced fragmentation of SC DNA results from a "clayting" double-strand break, involving, on an average, 180 bp.