Endonuclease is an enzyme that cleaves a specific nucleotide sequence at regions known as recognition sites, which are randomly distributed throughout a DNA or RNA strand. It plays a key role in DNA repair. It works by first examining the length of the DNA sequence looking for the specific recognition site within the strand. On finding the site, endonuclease binds to it and cleaves each of the DNA strands at specific points, removing groups of nucleotides on the strand. It does this by hydrolysis, which involves adding a water molecule to break the chemical bond between adjacent molecules.