Molecular biologists and other scientists are often interested in studying a particular sequence of DNA. The sequence can be anywhere from tens to thousands of bases in length and may be a part of a regulatory region for a gene or a gene that encodes a protein or RNA product. Scientists often need to obtain a large amount of purified DNA in order to analyze it. In order to do this the DNA sequence is inserted into a certain circular DNA “host” called a plasmid. The plasmid is then forced into bacteria such as E. coli by a process called transformation. The bacteria grow and divide in a solution nicknamed “broth” and the bacteria create numerous copies of the plasmid DNA. The bacteria are then harvested and the plasmid DNA is isolated, resulting in large amounts (micrograms to milligrams) of purified DNA that can be used in subsequent studies.
Many times a scientist requires more plasmid DNA than can be obtained with one transformation. Sometimes multiple transformations are performed, however this takes quite a bit of time and labor. Most laboratories get around this problem by creating “glycerol stocks” of the initial transformed bacteria. The glycerol stock consists of a mixture of the bacteria in broth with glycerol at an 80 percent to 20 percent ratio. The stock is then stored in a –80 ° C freezer, which does not kill the transformed bacteria. This is because of the glycerol, which prevents the water in the broth from forming ice crystals that can puncture and kill the bacterial cells. When a scientist requires more plasmid DNA, they simply scrape a bit of the glycerol stock into fresh broth, let the bacteria divide, and isolate the plasmid DNA from the resulting population. Glycerol stocks can be stored in the freezer for years if not decades without affecting the bacteria.