DNase I digestion patterns of chicken erythrocyte chromatin digested in whole nuclei in the presence of 5 mM MnCl2 or in the presence of 5 mM MgCl2 with increasing amounts of DNase I (10, 30, 100 and 300 U/mg DNA) on ice for 50 min. (A) 8% polyacrylamide gel in TBE, 7 M urea. (B) 2% agarose gel in glycine buffer. mar, restriction marker pAT/HpaII; m.n., micrococcal nuclease digest.