*12.Suppose that you want to compare the species of bacteria found in a polluted stream with the species found in an unpolluted stream. Traditionally, bacteria have been identified by growing them in the laboratory and comparing their physical and biochemical properties. You recognize that you will be unable to culture most of the bacteria that reside in the streams. How might you go about identifying the species in the two streams without culturing them in the laboratory?

*13.John Smith is a pig farmer. For the past five years, Smith has been adding vitamins and low doses of antibiotics to his pig food; he says that these supplements enhance the growth of the pigs. Within the past year, however, several of his pigs died from infections of common bacteria, which failed to respond to large doses of antibiotics. Can you explain the increased rate of mortality due to infection in Smith’s pigs? What advice might you offer Smith to prevent this problem in the future?

14.In Figure 7.8, what do the red and blue parts of the DNA labeled by balloon 6 represent?

*15. Austin Taylor and Edward Adelberg isolated some new strains of Hfr cells that they then used to map several genes in E. coli by using interrupted conjugation (A. L. Taylor and E. A. Adelberg. 1960. Genetics 45:1233–1243). In one experiment, they mixed cells of Hfr strain AB-312, which were xyl⁺ mtl⁺ mal⁺ met⁺ and sensitive to phage T6, with F⁻ strain AB-531, which was xyl⁻ mtl⁻ mal⁻ met⁻ and resistant to phage T6. The cells were allowed to undergo conjugation. At regular intervals, the researchers removed a sample of cells and interrupted conjugation by killing the Hfr cells with phage T6. The F⁻ cells, which were resistant to phage T6, survived and were then tested for the presence of genes transferred from the Hfr strain. The results of this experiment are shown in the accompanying graph. On the basis of these data, give the order of the xyl, mtl, mal, and met genes on the bacterial chromosome and indicate the minimum distances between them.

16.DNA from a strain of Bacillus subtilis with genotype a⁺ b⁺ c⁺ d⁺ e⁺ is used to transform a strain with genotype a⁻ b⁻ c⁻ d⁻ e⁻. Pairs of genes are checked for cotransformation, and the following results are obtained:

On the basis of these results, what is the order of the genes on the bacterial chromosome?

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17.DNA from a bacterial strain that is his⁺ leu⁺ lac⁺ is used to transform a strain that is his⁻ leu⁻ lac⁻. The following percentages of cells are transformed:

*18. Rollin Hotchkiss and Julius Marmur studied transformation in the bacterium Streptococcus pneumoniae (R. D. Hotchkiss and J. Marmur. 1954. Proceedings of the National Academy of Sciences of the United States of America 40:55–60). They examined four mutations in this bacterium: penicillin resistance (P), streptomycin resistance (S), sulfanilamide resistance (F), and the ability to utilize mannitol (M). They extracted DNA from strains of bacteria with different combinations of different mutations and used this DNA to transform wild-type bacterial cells (P⁺ S⁺ F⁺ M⁺). The results from one of their transformation experiments are shown below.

*19.Two mutations that affect plaque morphology in phages (a⁻ and b⁻) have been isolated. Phages carrying both mutations (a⁻ b⁻) are mixed with wild-type phages (a⁺ b⁺) and added to a culture of bacterial cells. Subsequent to infection and lysis, samples of the phage lysate are collected and cultured on bacterial cells. The following numbers of plaques are observed:

What is the frequency of recombination between the a and b genes?

*20. T. Miyake and M. Demerec examined proline-requiring mutations in the bacterium Salmonella typhimurium (T. Miyake and M. Demerec. 1960. Genetics 45:755–762). On the basis of complementation studies, they found four proline auxotrophs: proA, proB, proC, and proD. To determine whether proA, proB, proC, and proD loci were located close together on the bacterial chromosome, they conducted a transduction experiment. Bacterial strains that were proC⁺ and had mutations at proA, proB, or proD were used as donors. The donors were infected with bacteriophages, and progeny phages were allowed to infect recipient bacteria with genotype proC⁻ proA⁺ proB⁺ proD⁺. The recipient bacteria were then plated on a selective medium that allowed only proC⁺ bacteria to grow. After this, the proC⁺ transductants were plated on selective media to reveal their genotypes at the other three pro loci. The following results were obtained:

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*21.A geneticist isolates two mutations in a bacteriophage. One mutation causes clear plaques (c), and the other produces minute plaques (m). Previous mapping experiments have established that the genes responsible for these two mutations are 8 m.u. apart. The geneticist mixes phages with genotype c⁺ m⁺ and genotype c⁻ m⁻ and uses the mixture to infect bacterial cells. She collects the progeny phages and cultures a sample of them on plated bacteria. A total of 1000 plaques are observed. What numbers of the different types of plaques (c⁺ m⁺, c⁻ m⁻, c⁺ m⁻, c⁻ m⁺) should she expect to see?

*22.E. coli cells are simultaneously infected with two strains of phage λ. One strain has a mutant host range, is temperature sensitive, and produces clear plaques (­genotype h st c); another strain carries the wild-type alleles (genotype h⁺ st⁺ c⁺). Progeny phages are collected from the lysed cells and are plated on bacteria. The numbers of different progeny phages are as follows:

23.A donor strain of bacteria with alleles a⁺ b⁺ c⁺ is infected with phages to map the donor chromosome using generalized transduction. The phage lysate from the bacterial cells is collected and used to infect a second strain of bacteria that are a^– b^– c⁻. Bacteria with the a⁺ gene are selected, and the percentages of cells with cotransduced b⁺ and c⁺ genes are recorded.

Is the b or c gene closer to a? Explain your reasoning.

24.For the H1N1 influenza virus shown at the bottom of Figure 7.29, viruses from which organism contributed the most RNA to the virus?