In order to separate the DNA from the remaining cell materials, you must break open the cell membrane and the nuclear membrane. You will use heat to lyse² the cells by heating your sample to boiling. When heated, the bonds holding the cells' lipid bilayer together break down, releasing the cell contents. Certain detergent agents can also be used to break down the cell membranes and release cell contents. These are commonly used in the laboratory.

² a process of disintegration or dissolution (as of cells) - (http://www.merriamwebster.com)