In Figure 5-2, in which of the four processes shown can a complete bacterial genome be transferred from one cell to another?

In Figure 5-3, if the concentration of bacterial cells in the original suspension is 200/ml and 0.2 ml is plated onto each of 100 petri dishes, what is the expected average number of colonies per plate?

In Figure 5-5,

In Figure 5-10c, what do the yellow dots represent?

In Figure 5-11, which donor alleles become part of the recombinant genome produced?

In Figure 5-12,

In Figure 5-14, which is the last gene to be transferred into the F-from each of the five Hfr strains?

In Figure 5-15, how are each of the following genotypes produced?

In Figure 5-17, how many crossovers are required to produce a completely prototrophic exconjugant?

In Figure 5-18c, why is the crossover shown occurring in the orange segments of DNA?

In Figure 5-19, how many different bacterial species are shown as having contributed DNA to the plasmid pk214?

In Figure 5-25, can you point to any phage progeny that could transduce?

In Figure 5-28, what are the physical features of the plaques of recombinant phages?

In Figure 5-29, do you think that b⁺ could be transduced instead of a⁺? As well as a⁺?

In Figure 5-30, which genes show the highest frequencies of cotransduction?

In Figure 5-32, what do the half-red, half-blue segments represent?

In Figure 5-33, which is the rarest λ genotype produced in the initial lysate?

In Figure 5-38, precisely which gene is eventually identified from the genome sequence?

Describe the state of the F factor in an Hfr, F⁺, and F⁻ strain.

How does a culture of F⁺ cells transfer markers from the host chromosome to a recipient?

With respect to gene transfer and the integration of the transferred gene into the recipient genome, compare

Why is generalized transduction able to transfer any gene, but specialized transduction is restricted to only a small set?

A microbial geneticist isolates a new mutation in E. coli and wishes to map its chromosomal location. She uses interrupted-mating experiments with Hfr strains and generalized-transduction experiments with phage P1. Explain why each technique, by itself, is insufficient for accurate mapping.

In E. coli, four Hfr strains donate the following markers, shown in the order donated:

All these Hfr strains are derived from the same F⁺ strain. What is the order of these markers on the circular chromosome of the original F⁺?

You are given two strains of E. coli. The Hfr strain is arg⁺ ala⁺ glu⁺ pro⁺ leu⁺ T^s; the F⁻ strain is arg⁻ ala⁻ glu⁻ pro⁻ leu⁻ T^r. All the markers are nutritional except T, which determines sensitivity or resistance to phage T1. The order of entry is as given, with arg⁺ entering the recipient first and T^s last. You find that the F⁻ strain dies when exposed to penicillin (pen^s), but the Hfr strain does not (pen^r). How would you locate the locus for pen on the bacterial chromosome with respect to arg, ala, glu, pro, and leu? Formulate your answer in logical, well-explained steps, and draw explicit diagrams where possible.

A cross is made between two E. coli strains: Hfr arg⁺ bio⁺ leu⁺ × F⁻ arg⁻ bio⁻ leu⁻. Interrupted mating studies show that arg⁺ enters the recipient last, and so arg⁺ recombinants are selected on a medium containing bio and leu only. These recombinants are tested for the presence of bio⁺ and leu⁺. The following numbers of individuals are found for each genotype:

Linkage maps in an Hfr bacterial strain are calculated in units of minutes (the number of minutes between genes indicates the length of time that it takes for the second gene to follow the first in conjugation). In making such maps, microbial geneticists assume that the bacterial chromosome is transferred from Hfr to F⁻ at a constant rate. Thus, two genes separated by 10 minutes near the origin end are assumed to be the same physical distance apart as two genes separated by 10 minutes near the F⁻ attachment end. Suggest a critical experiment to test the validity of this assumption.

A particular Hfr strain normally transmits the pro⁺ marker as the last one in conjugation. In a cross of this strain with an F⁻ strain, some pro⁺ recombinants are recovered early in the mating process. When these pro⁺ cells are mixed with F⁻ cells, the majority of the F⁻ cells are converted into pro⁺ cells that also carry the F factor. Explain these results.

F′ strains in E. coli are derived from Hfr strains. In some cases, these F′ strains show a high rate of integration back into the bacterial chromosome of a second strain. Furthermore, the site of integration is often the site occupied by the sex factor in the original Hfr strain (before production of the F′ strains). Explain these results.

You have two E. coli strains, F⁻ str^s ala⁻ and Hfr str^s ala⁺, in which the F factor is inserted close to ala⁺. Devise a screening test to detect strains carrying F′ ala⁺.

Five Hfr strains A through E are derived from a single F ⁺ strain of E. coli. The following chart shows the entry times of the first five markers into an F⁻ strain when each is used in an interrupted-conjugation experiment:

Streptococcus pneumoniae cells of genotype str^s mtl⁻ are transformed by donor DNA of genotype str^r mtl⁺ and (in a separate experiment) by a mixture of two DNAs with genotypes str^r mtl⁻ and str^s mtl⁺. The accompanying table shows the results.

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Recall that, in Chapter 4, we considered the possibility that a crossover event may affect the likelihood of another crossover. In the bacteriophage T4, gene a is 1.0 m.u. from gene b, which is 0.2 m.u. from gene c. The gene order is a, b, c. In a recombination experiment, you recover five double crossovers between a and c from 100,000 progeny viruses. Is it correct to conclude that interference is negative? Explain your answer.

E. coli cells were infected with two strains of T4 virus. One strain is minute (m), rapid lysis (r), and turbid (t); the other is wild type for all three markers. The lytic products of this infection were plated and classified. The resulting 10,342 plaques were distributed among eight genotypes as follows:

With the use of P22 as a generalized transducing phage grown on a pur⁺ pro⁺ his⁺ bacterial donor, a recipient strain of genotype pur⁻ pro⁻ his⁻ was infected and incubated. Afterward, transductants for pur⁺, pro⁺, and his⁺ were selected individually in experiments I, II, and III, respectively.

Although most λ-mediated gal⁺ transductants are inducible lysogens, a small percentage of these transductants in fact are not lysogens (that is, they contain no integrated λ). Control experiments show that these transductants are not produced by mutation. What is the likely origin of these types?

An ade⁺ arg⁺ cys⁺ his⁺ leu⁺ pro⁺ bacterial strain is known to be lysogenic for a newly discovered phage, but the site of the prophage is not known. The bacterial map is

The lysogenic strain is used as a source of the phage, and the phages are added to a bacterial strain of genotype ade ⁻ arg ⁻ cys ⁻ his⁻ leu⁻ pro⁻. After a short incubation, samples of these bacteria are plated on six different media, with the supplementations indicated in the following table. The table also shows whether colonies were observed on the various media.

(In this table, a plus sign indicates the presence of a nutrient supplement, a minus sign indicates that a supplement is not present, N indicates no colonies, and C indicates colonies present.)

In a generalized-transduction system using P1 phage, the donor is pur⁺ nad⁺ pdx⁻ and the recipient is pur⁻ nad⁻pdx⁺. The donor allele pur⁺ is initially selected after transduction, and 50 pur⁺ transductants are then scored for the other alleles present. Here are the results:

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In a generalized-transduction experiment, phages are collected from an E. coli donor strain of genotype cys⁺ leu⁺ thr⁺ and used to transduce a recipient of genotype cys⁻ leu⁻ thr⁻. Initially, the treated recipient population is plated on a minimal medium supplemented with leucine and threonine. Many colonies are obtained.

Deduce the genotypes of the following E. coli strains 1 through 4:

In an interrupted-conjugation experiment in E. coli, the pro gene enters after the thi gene. A pro⁺ thi⁺ Hfr is crossed with a pro⁻ thi⁻ F⁻ strain, and exconjugants are plated on medium containing thiamine but no proline. A total of 360 colonies are observed, and they are isolated and cultured on fully supplemented medium. These cultures are then tested for their ability to grow on medium containing no proline or thiamine (minimal medium), and 320 of the cultures are found to be able to grow but the remainder cannot.

A generalized transduction e xperiment uses a metE⁺ pyrD⁺ strain as donor and metE⁻ pyrD⁻ as recipient. metE⁺ transductants are selected and then tested for the pyrD⁺ allele. The following numbers were obtained:

Do these results suggest that these loci are closely linked? What other explanations are there for the lone “double”?

An argC⁻ strain was infected with transducing phage, and the lysate was used to transduce metF⁻ recipients on medium containing arginine but no methionine. The metF⁺ transductants were then tested for arginine requirement: most were argC⁺ but a small percentage were found to be argC⁻. Draw diagrams to show the likely origin of the argC⁺ and argC⁻ strains.

Four E. coli strains of genotype a⁺ b⁻ are labeled 1, 2, 3, and 4. Four strains of genotype a⁻ b⁺ are labeled 5, 6, 7, and 8. The two genotypes are mixed in all possible combinations and (after incubation) are plated to determine the frequency of a⁺ b⁺ recombinants. The following results are obtained, where M = many recombinants, L = low numbers of recombinants, and 0 = no recombinants:

On the basis of these results, assign a sex type (either Hfr, F⁺, or F⁻) to each strain.

An Hfr strain of genotype a⁺ b⁺ c⁺ d⁻ str^s is mated with a female strain of genotype a⁻ b⁻ c⁻ d⁺ str^r. At various times, mating pairs are separated by vigorously shaking the culture. The cells are then plated on three types of agar, as shown below, where nutrient A allows the growth of a⁻ cells; nutrient B, of b⁻ cells; nutrient C, of c⁻ cells; and nutrient D, of d⁻ cells. (A plus indicates the presence of streptomycin or a nutrient, and a minus indicates its absence.)

In the cross Hfr aro⁺ arg⁺ ery^r str^s × F⁻ aro⁻ arg⁻ ery^s str^r, the markers are transferred in the order given (with aro⁺ entering first), but the first three genes are very close together. Exconjugants are plated on a medium containing Str (streptomycin, to kill Hfr cells), Ery (erythromycin), Arg (arginine), and Aro (aromatic amino acids). The following results are obtained for 300 colonies isolated from these plates and tested for growth on various media: on Ery only, 263 strains grow; on Ery + Arg, 264 strains grow; on Ery + Aro, 290 strains grow; on Ery + Arg + Aro, 300 strains grow.

A bacterial transformation is performed with a donor strain that is resistant to four drugs, A, B, C, and D, and a recipient strain that is sensitive to all four drugs. The resulting recipient cell population is divided and plated on media containing various combinations of the drugs. The following table shows the results.

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You have two strains of λ that can lysogenize E. coli; their linkage maps are as follows:

The segment shown at the bottom of the chromosome, designated 1–2–3, is the region responsible for pairing and crossing over with the E. coli chromosome. (Keep the markers on all your drawings.)

You have three strains of E. coli. Strain A is F′ cys⁺ trp1/cys⁺ trp1 (that is, both the F′ and the chromosome carry cys⁺ and trp1, an allele for tryptophan requirement). Strain B is F⁻cys⁻ trp2 Z (this strain requires cysteine for growth and carries trp2, another allele causing a tryptophan requirement; strain B is lysogenic for the generalized transducing phage Z). Strain C is F⁻ cys⁺ trp1 (it is an F⁻ derivative of strain A that has lost the F′). How would you determine whether trp1 and trp2 are alleles of the same locus? (Describe the crosses and the results expected.)

A generalized transducing phage is used to transduce an a⁻ b⁻ c⁻ d⁻ e⁻ recipient strain of E. coli with an a⁺ b⁺ c⁺ d⁺ e⁺ donor. The recipient culture is plated on various media with the results shown in the following table. (Note that a⁻ indicates a requirement for A as a nutrient, and so forth.) What can you conclude about the linkage and order of the genes?

In 1965, Jon Beckwith and Ethan Signer devised a method of obtaining specialized transducing phages carrying the lac region. They knew that the integration site, designated att80, for the temperate phage ϕ80 (a relative of phage λ) was located near tonB, a gene that confers resistance to the virulent phage T1:

They used an F′ lac⁺ plasmid that could not replicate at high temperatures in a strain carrying a deletion of the lac genes. By forcing the cell to remain lac⁺ at high temperatures, the researchers could select strains in which the plasmid had integrated into the chromosome, thereby allowing the F′ lac to be maintained at high temperatures. By combining this selection with a simultaneous selection for resistance to T1 phage infection, they found that the only survivors were cells in which the F′ lac had integrated into the tonB locus, as shown here:

This result placed the lac region near the integration site for phage ϕ80. Describe the subsequent steps that the researchers must have followed to isolate the specialized transducing particles of phage ϕ80 that carried the lac region.

Wild-type E. coli takes up and concentrates a certain red food dye, making the colonies blood red. Transposon mutagenesis was used, and the cells were plated on food dye. Most colonies were red, but some colonies did not take up dye and appeared white. In one white colony, the DNA surrounding the transposon insert was sequenced, with the use of a DNA replication primer identical with part of the end of the transposon sequence, and the sequence adjacent to the transposon was found to correspond to a gene of unknown function called atoE, spanning positions 2.322 through 2.324 Mb on the map (numbered from an arbitrary position zero). Propose a function for atoE. What biological process could be investigated in this way, and what other types of white colonies might be expected?