5. Genetics of Bacteria and Viruses

Transduction

5. Genetics of Bacteria and Viruses

Transduction

2:35 minutes

Problem 24d

Textbook Question

The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.

In Experiment B, why are there no transductants with the genotype leu⁻ ala⁺?

Verified step by step guidance

Step 1: Understand the concept of generalized transduction. Generalized transduction occurs when a bacteriophage transfers random fragments of bacterial DNA from a donor strain to a recipient strain during infection. This process can result in the transfer of specific genes, such as leu⁺, phe⁺, or ala⁺, depending on the experimental selection criteria.

Step 2: Analyze the data from Experiment B. In this experiment, transductants are selected for ala⁺, meaning only recipient cells that acquire the ala⁺ gene from the donor strain are able to grow. The table shows that there are no transductants with the genotype leu⁻ ala⁺.

Step 3: Consider the genetic linkage between the leu and ala genes. Genetic linkage refers to the physical proximity of genes on the bacterial chromosome. If leu and ala are closely linked, it is likely that they are transferred together during transduction. This means that a recipient cell acquiring ala⁺ from the donor strain would also acquire leu⁺, resulting in the genotype leu⁺ ala⁺.

Step 4: Evaluate the absence of leu⁻ ala⁺ transductants. The lack of leu⁻ ala⁺ transductants suggests that the ala gene is not transferred independently of the leu gene. Instead, the transfer of ala⁺ is always accompanied by the transfer of leu⁺ due to their close genetic linkage.

Step 5: Conclude that the absence of leu⁻ ala⁺ transductants in Experiment B is due to the genetic linkage between leu and ala. This linkage ensures that any recipient cell acquiring ala⁺ from the donor strain also acquires leu⁺, preventing the formation of leu⁻ ala⁺ genotypes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Generalized Transduction

Generalized transduction is a process by which a bacteriophage (phage) transfers genetic material from one bacterium to another. During this process, the phage can accidentally package bacterial DNA instead of its own, which can then be introduced into a recipient bacterium upon infection. This mechanism allows for the transfer of any gene from the donor strain, making it a powerful tool for genetic studies.

Selection of Transductants

In genetic experiments, transductants are selected based on specific traits conferred by the donor strain's genes. The selection process involves growing the recipient strain in conditions that favor the expression of the desired traits, such as the ability to synthesize certain amino acids. The absence of certain genotypes among the transductants can indicate the efficiency of gene transfer and the linkage of genes on the chromosome.

Linkage and Genetic Mapping

Linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. In the context of the experiments, the absence of the leu⁻ ala⁺ genotype in Experiment B suggests that the genes for leucine and alanine biosynthesis are closely linked. This linkage can affect the frequency of recombination events, leading to certain combinations being less likely to occur in the transductants.

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Textbook Question

How do we know that bacteriophages recombine genetic material through transduction and that cell-to-cell contact is not essential for transduction to occur?

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Textbook Question

How is the frequency of cotransduction related to the relative positions of genes on a bacterial chromosome? Draw a map of three genes and describe the expected relationship of cotransduction frequencies to the map.

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Textbook Question

A triple-auxotrophic strain of E. coli having the genotype phe⁻ met⁻ ara⁻ is used as a recipient strain in a transduction experiment. The strain is unable to synthesize its own phenylalanine or methionine, and it carries a mutation that leaves it unable to utilize the sugar arabinose for growth. The recipient is crossed to a prototrophic strain with the genotype phe⁺ met⁺ ara⁺. The table below shows the selected marker and gives cotransduction frequencies for the unselected markers.

Use the cotransduction data to determine the order of these genes.

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Textbook Question

A triple-auxotrophic strain of E. coli having the genotype phe⁻ met⁻ ara⁻ is used as a recipient strain in a transduction experiment. The strain is unable to synthesize its own phenylalanine or methionine, and it carries a mutation that leaves it unable to utilize the sugar arabinose for growth. The recipient is crossed to a prototrophic strain with the genotype phe⁺ met⁺ ara⁺. The table below shows the selected marker and gives cotransduction frequencies for the unselected markers.

Identify the compounds present in each of the selective media.

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Textbook Question

The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.

Diagram the crossover events that form each of the transductants in Experiment A.

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Textbook Question

The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.

Determine the order of genes on the donor chromosome.

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Textbook Question

The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.

What compound or compounds are added to the minimal medium to select for transductants in Experiments A, B, and C?

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Textbook Question

Price et al. [(1999). J. Bacteriol. 181:2358–2362] conducted a genetic study of the toxin transport protein (PA) of Bacillus anthracis, the bacterium that causes anthrax in humans. Within the 2294-nucleotide gene in 26 strains they identified five point mutations—two missense and three synonyms—among different isolates. Necropsy samples from an anthrax outbreak in 1979 revealed a novel missense mutation and five unique nucleotide changes among ten victims. The authors concluded that these data indicate little or no horizontal transfer between different B. anthracis strains.

What is meant by 'horizontal transfer'?

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