Active chromatin structure.

Active gene sequences are present in the nucleosomal fraction of chromatin. Histones are attached to the DNA of transcribing non-ribosomal genes, and EM images suggest that their conformation is related to that of nucleosomes, or is readily converted to that structure on preparation for microscopy. Active nucleosomes do have, however, a changed conformation, since their DNA is more susceptible to endonucleolytic attack by DNase I. This alteration appears to reflect potential rather than actual transcriptional activity. Mechanisms for nucleosome disassembly to allow transcription without histone have been proposed. These involve alterations in histone-histone interactions without perturbation of histone-DNA contact.