THE fundamental structural unit of chromatin is a nucleoprotein complex containing two molecules each of the histones H2A, H2B, H3 and H4 in association with approximately 200 base pairs of DNA (for review see ref. 1). At least 85% of nuclear DNA is organised in these chromatin subunits or nucleosomes2, and it has been shown that both transcribed3–8 and nontranscribed9–12 DNA sequences are arranged in nucleosomes. The length of DNA in the nucleosome repeat unit varies between organisms and even between cell types in the same organism1. The origin of this variation in repeat length, ranging from 160 to 240 base pairs per nucleosome, is unknown. However, there is a general correlation between repeat lengths and transcriptional activity : transcriptionally active cells have shorter nucleosome repeat lengths than less active cells13. Thus, it has been suggested that the nucleosome repeat length of transcribed DNA may be different from that of nontranscribed sequences, thereby implicating nucleosome oranisation in gene regulation14. We report here that both transcribed and nontranscribed DNA sequences are organised in nucleosomes with the same repeat lengths.
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Molecular Biology Reports (1983)
Studies in heterochromatin DNA: accessibility of late replicating heterochromatin DNA in chromatin to micrococcal nuclease digestion