Figure 4: Cryogenic grinding is used to extract DNA from calcified tissues such as teeth. In a freezer mill a ferromagnetic plunger is oscillated back-and-forth in alternating electric current. Liquid nitrogen is used to cool the sample, which results in making it extremely brittle and also protects DNA from heat degradation. The tooth is reduced to a powder to increase surface area and expose trapped cells to biochemical agents that release DNA into solution [56].