DNase I is an endonuclease derived from bovine pancreas that will degrade double-stranded DNA in the presence of divalent cations, producing 3'-OH oligonucleotides. Mg+2 in the reaction solution causes the enzyme to produce nicks in double-stranded DNA, while in the presence on Mn+2, DNase I cleaves both strands of the DNA. DNase I is useful in nick translation for introducing single-stranded nicks that serve as primer sites for initiation of DNA synthesis and for cloning random DNA fragments by cleaving double-stranded DNA. It is also used in purifying RNA from cells or transcription mixtures.