Mutagens are chemical agents that cause changes in the genetic code which are then passed on to future generations of an organism. Mutations are usually chemical in nature and often carcinogenic, but may also be caused by physical damage produced by x rays or other causes. A mutation changes the activity of a gene. Mutations are frequent in lower forms of life and may help these organisms adapt to changes in their environments.
Proteins are composed of chains of amino acids. In the genetic code of deoxyribonucleic acid ( DNA ), a codon or three-base sequence codes for the placement of each amino acid; for example, the codon UUU places phenylalanine at that location in the protein and replacement of the third base with adenine results in the placement of leucine instead of phenylalanine. If a portion of the original code read … UUUACG …, deleting one of the uridine bases would cause that portion of the code to read … UUACG… ; the sequence UUA would then specify leucine. A point mutation changing one base might result in the formation of a different protein.
Mutations can occur by several mechanisms, such as replacing one nucleotide base with another or by adding or removing a base; they can also develop when a carcinogenic agent such as an aromatic hydrocarbon molecule is inserted between the strands of DNA, causing the code to be misread. Some chemical mutagens such as nitrites change one base into another, resulting in a new sequence of amino acids and the synthesis of a new protein. The modified protein might function normally or might not be useful at all, but it could be dangerous.
Many mutagenic agents are also carcinogenic, and the Ames test provides a quick method for screening foods and other substances for potential cancer-causing agents.
HOW DOES THE AMES TEST WORK?
The Ames test is a method for screening potential mutagens. The test uses auxotrophs (strains that have lost the ability to synthesize a needed substance) of Salmonella typhimurium that carry mutant genes, making them unable to synthesize histidine. They can live on media containing histidine, but die when the amino acid is depleted. The bacteria are especially sensitive to back mutations that reactivate the gene for the synthesis of histidine; exposure to mutagenic substances allows the bacteria to grow rapidly, developing large and numerous colonies.
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Voet, D.; Voet, J. G.; and Pratt, C. W. (2003). Biochemistry, 3rd edition. New York: Wiley.