As long ago as 1868, Frederich Miescher extracted a phosphate containing substance from pus cells (white blood cells), suspected it had something to do with inheritance, and called it nuclein . The molecular structure of this substance, renamed deoxyribonucleic acid (DNA) , was not solved until nearly 100 years later by James Watson, Francis Crick and Maurice Wilkins (1962 Nobel Prize), with significant contributions by Rosalind Franklin and Erwin Chargaff.

Today, extracting DNA from cells is a relatively simple procedure, facilitated by the availability of naturally-occurring enzymes that degrade away proteins, Ribonucleic acid (RNA) , and other cellular components, leaving the DNA behind (Figure 1-A). Laboratories purchase purified concentrates of these enzymes to extract very high quality DNA in large quantities. Purified DNA in a test tube resembles a translucent glob of mucous.

DNA can be extracted from almost any tissue type. A small number of live cells can also be used as seed stock to grow an immense number of cells in tissue culture. Such cells are grown in dishes or flasks at controlled temperatures and fed specific nutrients, eventually producing an identical population of cells (1-B). Thus, tissue culture provides an unlimited and renewable source of DNA.

To preserve cells grown in culture, they are frozen in liquid nitrogen at temperatures below -196 C°, where they can remain indefinitely. The freezing solution contains an inert cryopreservative that prevents the formation of crystals during freezing (1-C). Frozen cell cultures provide a long-term source of DNA, which is invaluable in cases where the availability of tissue or blood samples is limited, such as for endangered species.