Deoxyribonuclease I (DNase I) is an endonuclease that digests single- or double-stranded DNA. It recognizes and cleaves phosphodiester bonds to produce a single deoxynucleotide or single- or double-stranded oligodeoxynucleotide with 5'-phosphorylated and 3'-hydroxylated ends. The activity of DNase I depends on Ca ²⁺ , and DNase I can also be activated by divalent metal ions such as Mg ²⁺ and Mn ²⁺ . In the presence of Mg ²⁺ , DNase I nonspecifically recognizes and cleaves a double-stranded DNA at any site on either strand, and in the presence of Mn ²⁺ , it recognizes and cleaves almost the same sites on both strands of the DNA to produce DNA fragments with blunt ends or sticky ends with one or two nucleotide overhangs.