[Two types of fragmentation of total chromatin by DNAase I--an artifact or reality?]

The possibility of N+2N repeat (a nucleosomal-type repeat in which the even-numbered peaks dominate) being an artifact has been studied. The repeat results from digestion of chromatin of several rat cells by DNAase I. Endogenous nucleases are not shown to be involved in formation of the repeat. N+2N repeat is also formed during digestion of nuclei isolated from homogeneous lymphocyte populations indicating that the repeat is inherent of chromatin of distinct cells and is not the result of superimposition of different repeats arising from diverse tissue cells. We suppose the N+2N repeat to indicate the existence of the second type of total chromatin differing from the one giving rise to the dinucleosome repeat under the same conditions.