Mammalian chromatin substructure studies with the calcium–magnesium endonuclease and two-dimensional polyacrylamide-gel electrophoresis

The basic regularity of chromatin substructure that has been reported in rat liver chromatin (Hewish & Burgoyne, 1973b) was also detected in mouse chromatin. The regular series of DNA fragments produced by the action of Ca-Mg endonuclease on rat chromatin were studied further. The smallest single-stranded class has a molecular weight of approx. 45000-63000 and the smallest double-stranded class has a molecular weight of approx. 120000-150000. Studies of the substructure of the DNA fragments produced by the Ca-Mg endonuclease have shown that the regular series of double-stranded fragments have regular series of single-stranded fragments within them. It was concluded that the regular series of double-stranded fragments was probably a consequence of the regular series of single-stranded fragments. Digestion time-courses are presented for mouse and rat nuclear DNA.