Abstract: A method of determining activity of a nucleic acid ligase or a nucleic acid nuclease is described. This method comprises the steps of: (i) providing a nucleic acid molecule comprising a hairpin with a single-stranded loop and a double-stranded stem containing a target site for the nucleic acid ligase and/or the nucleic acid nuclease, wherein the nucleic acid molecule has a first end tethered to a surface and a second end remote from the first end, and wherein a detectable label is attached to the nucleic acid molecule either at the second end or between the target site and the second end; (ii) contacting the nucleic acid molecule with the nucleic acid ligase or the nucleic acid nuclease; and (iii) detecting the presence or absence of the detectable label, thereby determining activity of the nucleic acid ligase or the nucleic acid nuclease.