Solution

In order to cut the DNA with restriction enzymes, it needs to be in pure form, free from other macromolecules. Since the DNA is enclosed by the membranes, we have to break the cell open to release DNA and other macromolecules like RNA, proteins, polysaccharides, and lipids.
It is obtained by treating the bacterial cells/plant or animal tissue with enzymes such as lysozyme (bacteria), cellulase (plant cells) and chitinase (fungus). DNA is intertwined with proteins like histones.
RNA can be removed by treatment with ribonuclease while proteins can be removed by treatment with protease. Other molecules are removed by appropriate treatments and purified DNA ultimately precipitates out after the addition of chilled ethanol.