Identification of DNaseI Hypersensitive Sites Within Nuclei

The vast bulk of the genome of eukaryotic cells exists as a highly regular array of nucleosomes. A single nucleosome comprises 146 bp of DNA wrapped around a histone core particle. Arrays of nucleosomes usually exist as condensed chromatin fibers with a nucleosome repeat length of about 200 bp. The DNA within condensed chromatin is for the most part relatively inaccessible to proteins, such as nucleases. However, about 1% of the genome exists as discrete regions of decondensed chromatin termed DNaseI hypersensitive (DH) sites (1 ) which provide greatly increased access to factors that interact with DNA. DH sites represent local perturbations of the nucleosome array and probably arise via the reorganization of single nucleosomes within the chromatin fiber. Wherease DH sites exist as open regions of chromatin in nuclei, they are not typically hypersensitive to DNaseI within protein-free DNA.