Chromatin structure of eukaryotic genes : DNase I hypersensitive sites
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We have recently learned much about the organization of the eukaryotic genome by using nucleases as probes of the protein - DNA interactions of chromatin. In this paper we review the available information from our laboratory and others concerning the DNase I hypersensitive sites of chromatin. Such sites have been found at or near the 5' ends of many genes. Frequently there is a cluster of these sites before a given gene. In addition, a number of sites have been observed which are not 5' to any known transcript; these may play roles in DNA replication or gene rearrangement. The 5' DNase I hypersensitivesites appear during development before a given gene is expressed. In several cases these events are closely temporally linked, but in some instances 5' DNase I hypersensitivesites are apparent long before a gene is due to be transcribed. For example, many sites observed in the post-blastula embryo of Drosophila are established in the pre-plastula stage. Themolecular nature of DNase I hypersensitive sites is unclear. In some instances sites have been shown to be nucleosome free regions; however the pattern of association or proteins, if any, as well as the form of the DNA molecule, remain to be established.