DNA extraction is the removal of deoxyribonucleic acid, the blueprints for any organism's body and physiology, from its cells. Macromolecules are large molecules made up of chains of smaller, similar molecules. The nucleic acids that make up DNA are themselves macromolecules, but the same cells that contain the DNA also contain many other macromolecules that can interfere with its extraction.
Chains of Links
Proteins, lipids, carbohydrates and nucleic acids are all macromolecules and they're all present in cells. Macromolecules are polymers, long chains made up of many similarly structured, smaller monomers (or triglycerides in the case of lipids). Polymers use condensation to add monomers to their chains and hydrolysis to subtract them. DNA is made up of macromolecules of nucleic acids, but it's also surrounded by the many other macromolecules of lipids, proteins and carbohydrates that make up the cell and its cytoplasmic contents. One key to successful extraction is to break up other cellular macromolecules without breaking DNA.
Prokaryotic DNA of bacteria and archaea is contained inside one cellular membrane, while eukaryotic animal, plant, fungal and protozoan DNA is hidden inside two packages, the nuclear membrane and the cell's wall or membrane. Cell membranes are made up of fats, proteins and carbohydrates, and cell walls are composed of carbohydrates and proteins, all macromolecules. If left intact in a sample, they can trap DNA. The first step in DNA extraction is the physical breakage, or lysis, of cell walls and membranes, which dumps the cellular contents, including the DNA, into solution. This is followed by emulsification, where salts and detergents plus gentle, lengthy mixing break down the membrane proteins, fats and carbohydrates further.
Once released from the outer membranes, DNA is still surrounded by proteins. These protein macromolecules can also trap the DNA and keep it from precipitating out of the sample. Enzymes are used to cut the protein envelopes open, but must do so without breaking the DNA chains because broken DNA won't spool out of solution during the last extraction step. Since DNA is much larger and more stable than other cellular macromolecules, its chains can usually be preserved simply by using weak enzymes and keeping the sample cool.
Inner and Outer Intruders
There are additional proteins and carbohydrates floating around inside cells' cytoplasm that can cause DNA in a sample to clump instead of forming the strings necessary for successful extraction. These are the materials that make up cellular components such as organelles, and extraction uses alcohol such as ethanol to separate them from DNA. Finally, samples can become contaminated with nucleic acid macromolecules of DNA and RNA from other environmental sources. Careful handling can limit this contamination.
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