Hot water does not release DNA from cell nuclei. Instead hot water can damage and denature the delicate components inside cells (including the nucleus and DNA), especially if a very high temperature of almost boiling was used.

To extract DNA, lysis buffers with specific composition and concentrations of salts and detergents or enzymes such as proteinase K, and sometimes a DNA extraction kit with columns, are instead widely used for safely releasing (not "freeing") genomic DNA from white blood cells.