Membranes are primarily composed of a bilayer of lipids (fats).
In addition to lipids, there are proteins, carbohydrates, and combinations of the three within the bilayer, but the barrier that separates the cell from its exterior environment is established by the lipids. It is called a bilayer because, as you can see in the above image, it is formed by sheets of lipids in opposite conformations. How does this occur? By the nature of the lipid - having a polar, hydrophilic head, and a long, hydrophobic. The cells in a sample are separated from each other, often by a physical means such as grinding or vortexing, and put into a solution containing salt. The positively charged sodium ions in the salt help protect the negatively charged phosphate groups that run along the backbone of the DNA.
A detergent is then added. The detergent breaks down the lipids in the cell membrane and nuclei. DNA is released as these membranes are disrupted. To get a clean sample of DNA, it’s necessary to remove as much of the cellular debris as possible. This can be done by a variety of methods. Often a protease ( protein enzyme) is added to degrade DNA-associated proteins and other cellular proteins. Alternatively, some of the cellular debris can be removed by filtering the sample. Finally, ice-cold alcohol (either ethanol or isopropanol) is carefully added to the DNA sample. DNA is soluble in water but insoluble in the presence of salt and alcohol. By gently stirring the alcohol layer with a sterile pipette, a precipitate becomes visible and can be spooled out. If there is lots of DNA, you may see a stringy, white precipitate. The DNA sample can now be further purified (cleaned). It is then resuspended in a slightly alkaline buffer and ready to use.
