CONCEPT DEVELOPMENT #4
DNA EXTRACTION
Students' Background Reading
DNA is found in the cells of all living things. When living organisms are exposed to excess UV radiation, it is the DNA that becomes damaged. Visualizing how DNA actually looks will help you understand how damage can occur. DNA is like an immensely long, uncooked piece of spaghetti; it can bend just so far before it breaks. It looks flexible on the macroscopic scale because it is so long in comparison with its thickness. Even though DNA is strong, it is stiff and brittle and must be handled gently.
How can a cell be opened to extract DNA? Cell membranes are made of proteins and fats. Just as detergent gently dissolves fats in a frying pan, a little detergent gently dissolves cell membranes:
As the cell membranes dissolve, the cell contents flow out into the liquid where the cells were growing. Since bacterial cells do not have a nucleus, their DNA, the dissolved cell membranes and other contents of the cell become suspended and float in a cell "soup." However, DNA is soluble in water and cannot be seen in this soup. DNA is insoluble in alcohol, however, and will precipitate upward if a layer of alcohol is gently added to the cell soup.