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Mutants can be selected using either direct techniques or indirect techniques such as replica plating. Penicillin enrichment can often help in the isolation of auxotrophic mutants by killing multiplying cells. Mutants that do not involve the biosynthesis of growth factors can be isolated using other techniques such as mutant screens and incubating cells at low temperatures.

■ Distinguish between direct and indirect selection.

■ When does penicillin enrichment not work?

■ The Ames test measures the reversion of a mutant gene. Would it be just as good to test for the generation of a mutant gene rather than its reversion? Explain.

■ How could you demonstrate by replica plating that the environment selects but does not mutate genes in bacteria?

202 Chapter 8 Bacterial Genetics

Bacteria requiring histidine ' for growth (His-)

Diffusion of added chemical

Bacteria requiring histidine ' for growth (His-)

Diffusion of added chemical

If chemical is a mutagen, many colonies form

If chemical is not a mutagen, far fewer colonies form

If chemical is a mutagen, many colonies form

If chemical is not a mutagen, far fewer colonies form

(a) Test plates

Few colonies form

(b) Control plate

Figure 8.12 Ames Test to Screen for Mutagens (a)The chemical will increase the frequency of reversion of His: to His+ cells if it is a mutagen and, therefore, most likely a carcinogen. (b)The control plate contains only the liquid in which the suspected mutagen is dissolved.

Figure 8.12 Ames Test to Screen for Mutagens (a)The chemical will increase the frequency of reversion of His: to His+ cells if it is a mutagen and, therefore, most likely a carcinogen. (b)The control plate contains only the liquid in which the suspected mutagen is dissolved.

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