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Deoxyribonuclease from beef pancreas, DNase I, was first crystallized by Kunitz. It is an endonuclease which splits phosphodiester linkages, preferentially adjacent to a pyrimidine nucleotide yielding 5'-phosphate terminated polynucleotides with a free hydroxyl group on position 3'. The average chain of limit digest is a tetranucleotide. DNase I acts upon single chain DNA, and upon double-stranded DNA and chromatin. In the latter case, although histones restrict susceptibility to nuclease action, over a period of time nearly all chromatin DNA is acted upon. According to Mirsky and Silverman, this could result from the looseness of histone attachment to DNA. They found that lysine-rich histones more effectively block DNase access to DNA than arginine-rich histone
