Ames observed that mutations in these essential genes could enable or disable the growth of bacteria on a petri dish. A strain of Salmonella normally unable to grow on galactose, say, could acquire a gene mutation that enabled this growth. Once growth-enabled, a single bacterium would form a minuscule colony on a petri dish. By counting the number of growth-enabled colonies formed, Ames could quantify the mutation rate in any experiment. Bacteria exposed to a certain substance might produce six such colonies, while bacteria exposed to another substance might produce sixty. This second
...more
