The Basics of DNA Extraction

You’ve probably heard of the Genetic Code or the Blueprint of Life; these terms refer to DNA. All living things, including animals, plants, and bacteria, have DNA in their cells. DNA is a very long molecule made up of a chain of nucleotides and the order of these nucleotides is what makes organisms similar to others of their species and yet different as individuals. Genes are sections within this long DNA molecule.

In order to study DNA, you first have to get it out of the cell. In eukaryotic cells, such as human and plant cells, DNA is organized as chromosomes in an organelle called the nucleus. Bacterial cells have no nucleus. Their DNA is organized in rings or circular plasmids, which are in the cytoplasm. The DNA extraction process frees DNA from the cell and then separates it from cellular fluid and proteins so you are left with pure DNA.

The three basic steps of DNA extraction are 1) lysis, 2) precipitation, and 3) purification.

Step 1: Lysis
In this step, the cell and the nucleus are broken open to release the DNA inside and there are two ways to do this. First, mechanical disruption breaks open the cells. This can be done with a tissue homogenizer (like a small blender), with a mortar and pestle, or by cutting the tissue into small pieces. Mechanical disruption is particularly important when using plant cells because they have a tough cell wall. Second, lysis uses detergents and enzymes such as Proteinase K to free the DNA and dissolve cellular proteins.

Step 2: Precipitation
When you complete the lysis step, the DNA has been freed from the nucleus, but it is now mixed with mashed up cell parts. Precipitation separates DNA from this cellular debris. First, Na+ ions (sodium) neutralize the negative charges on the DNA molecules, which makes them more stable and less water soluble. Next, alcohol (such as ethanol or isopropanol) is added and causes the DNA to precipitate out of the aqueous solution because it is not soluble in alcohol.

Step 3: Purification
Now that DNA has been separated from the aqueous phase, it can be rinsed with alcohol to remove any remaining unwanted material and cellular debris. At this point the purified DNA is usually re-dissolved in water for easy handling and storage.

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Targeted Alaska Grade Level Expectations
[9] SC1.1 The student demonstrates an understanding of how science explains changes in life forms over time, including genetics, heredity, the process of natural selection, and biological evolution by recognizing that all organisms have chromosomes made of DNA and that DNA determines traits.