Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

5. Genetics of Bacteria and Viruses

Bacterial Conjugation

Genetics

5. Genetics of Bacteria and Viruses

Bacterial Conjugation

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1:32 minutes

Problem 11

Textbook Question

Explain the observations that led Zinder and Lederberg to conclude that the prototrophs recovered in their transduction experiments were not the result of F⁺ mediated conjugation.

Verified step by step guidance

Understand the context: Zinder and Lederberg were studying bacterial genetics and discovered a process called transduction, where bacteriophages transfer genetic material between bacteria. They were investigating whether the prototrophs (bacteria that regained the ability to synthesize essential nutrients) observed in their experiments were due to transduction or F⁺ mediated conjugation.

Review the experimental setup: They used two auxotrophic strains of Salmonella typhimurium (bacteria that cannot synthesize certain nutrients) and placed them in a U-tube separated by a fine filter. This filter allowed the passage of small molecules and viruses but not bacterial cells.

Analyze the key observation: Prototrophs were recovered on the other side of the filter, even though the bacterial cells could not physically come into contact. This ruled out F⁺ mediated conjugation, as conjugation requires direct cell-to-cell contact through a pilus.

Consider the role of bacteriophages: Zinder and Lederberg hypothesized that bacteriophages were responsible for transferring genetic material between the bacterial strains. This process, later termed transduction, explained how genetic exchange occurred without direct contact.

Conclude the reasoning: The inability of bacterial cells to physically interact, combined with the recovery of prototrophs, led Zinder and Lederberg to conclude that the genetic exchange was mediated by bacteriophages and not by F⁺ mediated conjugation.

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Key Concepts

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

Transduction

Transduction is a process of genetic transfer in bacteria where bacteriophages (viruses that infect bacteria) transfer DNA from one bacterium to another. This mechanism allows for the introduction of new genetic material, leading to genetic variation. Zinder and Lederberg's experiments utilized this method to demonstrate how genetic traits could be transferred without direct contact between bacterial cells.

Conjugation

Conjugation is a form of horizontal gene transfer in bacteria that involves the direct transfer of DNA from one bacterium to another through a physical connection called a pilus. This process typically involves F⁺ (fertility factor positive) bacteria that can donate genetic material to F⁻ (fertility factor negative) bacteria. Zinder and Lederberg's findings indicated that the prototrophs they observed were not produced through this mechanism, as they lacked the necessary conditions for conjugation.

Prototrophs

Prototrophs are organisms that can synthesize all the compounds they need for growth from basic nutrients, unlike auxotrophs, which require additional growth factors due to mutations. In Zinder and Lederberg's experiments, the recovery of prototrophs suggested that genetic material had been transferred and expressed, leading to the restoration of the ability to synthesize essential compounds. This observation was critical in distinguishing transduction from conjugation in their research.

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Related Practice

Textbook Question

Describe the basis for chromosome mapping in the Hfr x F⁻ crosses.

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Textbook Question

Why are the recombinants produced from an Hfr x F⁻ cross rarely, if ever, F⁺?

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Textbook Question

Describe the origin of F' bacteria and merozygotes.

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Textbook Question

Among the mechanisms of gene transfer in bacteria, which one is capable of transferring the largest chromosome segment from donor to recipient? Which process generally transfers the smallest donor segments to the recipient? Explain your reasoning for both answers.

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Textbook Question

Hfr strains that differ in integrated F factor orientation and site of integration are used to construct consolidated bacterial chromosome maps. The data below show the order of gene transfer for five strains.

Hfr Strain Order of Gene Transfer (First → Last)

Hfr A oriT–thr–leu–azi–ton–pro–lac–ade

Hfr B oriT–mtl–xyl–mal–str–his

Hfr C oriT–ile–met–thi–thr–leu–azi–ton

Hfr D oriT–his–trp–gal–ade–lac–pro–ton

Hfr E oriT–thi–met–ile–mtl–xyl–mal–str

Draw a consolidated map of the bacterial chromosome. (Hint: Begin by placing the insertion site for Hfr A at the 2 o'clock position and arranging the genes thr–leu–azi- . . . in clockwise order.)

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Textbook Question

Hfr Strain Order of Gene Transfer (First → Last)

Hfr A oriT–thr–leu–azi–ton–pro–lac–ade

Hfr B oriT–mtl–xyl–mal–str–his

Hfr C oriT–ile–met–thi–thr–leu–azi–ton

Hfr D oriT–his–trp–gal–ade–lac–pro–ton

Hfr E oriT–thi–met–ile–mtl–xyl–mal–str

Identify the overlaps between Hfr strains. Identify the orientations of integrated F factors relative to one another.

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Textbook Question

Five Hfr strains from the same bacterial species are analyzed for their ability to transfer genes to F⁻ recipient bacteria. The data shown below list the origin of transfer (oriT) for each strain and give the order of genes, with the first gene on the left and the last gene on the right. Use the data to construct a circular map of the bacterium.

Hfr Strain Genes Transferred

Hfr 1 oriT met ala lac gal

Hfr 2 oriT met leu thr azi

Hfr 3 oriT gal pro trp azi

Hfr 4 oriT leu met ala lac

Hfr 5 oriT trp azi thr leu met

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Textbook Question

An interrupted mating study is carried out on Hfr strains 1, 2, and 3 identified in Problem 17. After conjugation is established, a small sample of the mixture is collected every minute for 20 minutes to determine the distance between genes on the chromosome. Results for each of the three Hfr strains are shown below. The total duration of conjugation (in minutes) is given for each transferred gene.

Hfr strain 1 oriT met ala lac gal

Duration (min) 0 2 8 13 17

Hfr strain 2 oriT met leu thr azi

Duration (min) 0 2 7 10 17

Hfr strain 3 oriT gal pro trp azi

Duration (min) 0 3 8 14 19

Using the chromosome map you prepared in answer to Problem 17, determine the distance in minutes between each gene on the map.

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