Question

1.What are some advantages and disadvantages of the Ames test over tests for carcinogens that use animal models?

2.Define the following terms:

a.Mutagen

b.Carcinogen

c.Auxotroph

d.Prototroph

e.Back-mutation

Answer 1

1. Ames test is a method which uses strains of bacterium Salmonella typhimurium to evaluate the carcinogenic effect of various carcinogen and toxin.These strains of bacteria carry frameshift or point mutation in genes involved in histidine synthesis, mutaion in lipopolysaccharide synthesis making cell wall more permeable, mutation inexcision repair system and and are auxotrophic ie, they require histidine for growth but cannot produce it .This method check the ability of the substance to return these strains into prototrophic strains ie. they can grow without added histidine in the media. The bacteria is spread on agar plate with little amount of added histidine so that some colonies grow and then given chance to mutate upon adding the substance to be tested , so further after depletion of histidine only those bacteria which have become phototrophic will grow and form clonies on the plate.The mutagenicity of carcinogenicity of the substance is proportional to number of colonies.

Advatages: This test reveal the carcinogenicity of a substance by finding its mutagenic capability.A large number of chemical which were commercially used and passed animal test were found to be highly carcinogenic through Ames test. Further they were withdrawn from the market.It is also used in initial screening of potential drug.

Limitation:

1.Salmonella typhimurium is a prokaryote so it cannot be a good model for humans as they are eukaryotes.

2. Rat liver enzyme is used in this test for mimicking mammalian metabolic tissue but there is great difference in metabolism of rat and mammals so it can affect the tested substance mutagenicity.

3. It is possible that substance found to be mutagen by Ames test may not be carcinogenic so other tests are needed to confirm carcinogenicity.

2. a) Mutagen is a substance which produces changes in the genetic make up of an organism usually DNA. It can be any physical or chemical agent.

b) Carcinogen is a substance which causes cancer in an organism. Cancer is uncontrolled proliferation of cells.It can be due to change in genetic composition but not all mutagen lead to carcinogenesis.

c) Auxotroph is an organism which is unable to synthesize compounds necessary for their growth and hence their presence is required in the media used to grow them .

d) Prototroph have the same nutritional requirements as auxotroph but they can synthesize those nutrients by themselves and donot require its presence in the growth media.

e) Back mutation is a mutation that cause the reversion of change in nucleotide sequence of the mutant gene and hence the original sequence and original phenotype of the organism is restored.