 E. coli cells containing fragments of human DNA, or any other type of DNA, can be stored in freezers indefinitely. When scientists need to retrieve DNA from the library, they simply revive the cells by bringing them back up to 37 degrees centigrade, gut temperature. The E. coli cells act as copiers, producing many copies of the human DNA sequence that they contain. To prepare to sequence DNA, a clone of cells containing the same bit of human DNA is released into a rich warm broth. The cells are shaken vigorously to provide them with air. This causes them to divide rapidly, about once every half hour. After incubating for just a single night, one-third of a teaspoon of broth contains billions of E. coli cells, and so, billions of copies of the particular fragment of human DNA they contain.