1. Chapter 8 Outline
8.1 Genetic Analysis of Bacteria Requires Special Approaches and Methods, 201
8.2 Viruses Are Simple Replicating Systems Amenable to Genetic Analysis, 218

2. 8.1 Genetic Analysis of Bacteria Requires Special Approaches and Methods
Techniques for the Study of Bacteria
Prototrophic – wild type
Auxotrophic – mutant type
Minimum medium: only required by prototrophic bacteria
Complete medium: contain all substance required by all bacteria, including auxotrophic bacteria

3. The Bacterial Genome Mostly single, circular DNA molecule/chromosome
Single copy of most genes
No dominant or recessive
Expressed or not
Little to no space between genes
No nucleus, so no separation of transcription and translation

4. 8.5 Most bacterial cells possess a single, circular chromosome, shown here emerging from a ruptured bacterial cell. [David L. Nelson and Michael M. Cox, Lehninger Principles of Biochemistry, 4th ed. (New York:Worth Publishers, 2004), from Huntington Potter and David Dressler, Harvard Medical School, Department of Neurobiology.]

5. The Bacterial Genome Extra chromosome, small circular DNA
Episomes – plasmids / F (fertility) factor

6. 8. 8 The F factor, a circular episome of E
8.8 The F factor, a circular episome of E. coli, contains a number of genes that regulate transfer into the bacterial cell, replication, and insertion into the bacterial chromosome. Replication is initiated at oriV. Insertion sequences (see Chapter 11) IS3 and IS2 control insertion into the bacterial chromosome and excision from it.

7. Horizontal Transfer Movement of DNA from one bacterial cell to another
Results in change of individual cell within its life
Different from Vertical Transmission (inheritance)
3 main mechanisms
Conjugation
Transformation
Transduction

8. 8.10 Lederberg and Tatum’s experiment demonstrated that bacteria undergo genetic exchange.

9. Gene Transfer in Bacteria
Conjugation:
Direct transfer via connection tube, one-way traffic from donor cells to recipient cells.
It is not a reciprocal exchange of genetic information.

10. Gene Transfer in Bacteria
Conjugation:
F+ cells donor cells containing F factor
F− cells recipient cells lacking F factor
Sex Pilus – connection tube

11. 8.12 A sex pilus connects F⁺ and F⁻ cells during bacterial conjugation. E. coli cells in conjugation. [Dr. Dennis Kunkel/Phototake.]

12. Gene Transfer in Bacteria
Conjugation:
Hfr cells: (high-frequency strains) – donor cells with F factor integrated into the donor bacterial chromosome
F prime (F′) cells: contain F plasmid carrying some bacterial genes
Merozygotes: partial diploid bacterial cells containing F plasmid carrying some bacterial genes

14. Gene Transfer in Bacteria
Conjugation:
Mapping bacterial genes with interrupted Conjugation: Distances between genes are measured by the time required for DNA transfer from Hfr cells to F− cells.

15. 8.17 Jacob and Wollman used interrupted conjugation to map bacterial genes.

16. 8.17 (part 2) Jacob and Wollman used interrupted conjugation to map bacterial genes.

17. Gene Transfer in Bacteria
Conjugation:
Natural Gene Transfer and Antibiotic Resistance
Antibiotic resistance comes from the actions of genes located on R plasmids that can be transferred naturally.

18. Gene Transfer in Bacteria
Conjugation:
Natural Gene Transfer and Antibiotic Resistance
Antibiotic resistance comes from the actions of genes located on R plasmids that can be transferred naturally.
R plasmids have evolved in the past 60 years since the beginning of widespread use of antibiotics.
The transfer of R plasmids is not restricted to bacteria of the same or even related species.

19. Gene Transfer in Bacteria
Transformation:
A bacterium takes up DNA from the medium.
Recombination takes place between introduced genes and the bacterial chromosome.

20. Gene Transfer in Bacteria
Transformation:
Competent cells: cells that take up DNA
Transformants: cells that receive genetic material
Cotransformed: cells that are transformed by two or more genes

21. 8.20 Genes can be transferred between bacteria through transformation.

22. 8.20 Genes can be transferred between bacteria through transformation.

23. Bacterial Genome Sequences:
1 ~ 4 million base pairs of DNA
Horizontal Gene Transfer:
Genes can be passed between individual members of different species by nonreproductive mechanisms.
Model Genetic Organism:
The bacterium Esherichia coli

24. 8.22 Escherichia coli is a model genetic organism.

25. Gene Transfer in Bacteria
Transduction:
Bacterial viruses (bacteriophage) carry DNA from one bacterium to another.
Transduction usually occurs between bacteria of the same or closely related species.

26. 8.9 Conjugation, transformation, and transduction are three processes of gene transfer in bacteria. For the transferred DNA to be stably inherited, all three processes require the transferred DNA to undergo recombination with the bacterial chromosome.

27. 8.2 Viruses Are Simple Replicating Systems Amenable To Genetic Analysis
Virus: Replicating structure (DNA/RNA) + protein coat
Bacteriophage: bacterial infection viruses
Virulent phages: reproduce through the lytic cycle, and always kill the host cells.
Temperate phages: inactive prophage–phage DNA integrates into bacterial chromosomes.

28. 8.24 Bacteriophages have two alternative life cycles: lytic and lysogenic.

29. Techniques for the study of bacteriophages
Transduction: using phage to map bacterial genes
General transduction: any genes can be transferred
Specialized transduction: only a few genes can be transferred

30. 8.26 The Lederberg and Zinder experiment.

31. 8. 29 Bacteria can exchange genes through specialized transduction
8.29 Bacteria can exchange genes through specialized transduction. Segments 1, 2, and 3 represent genes on the phage chromosome.

32. Techniques for the study of bacteriophages
Gene Mapping in Phages

33. 8. 30 Hershey and Rotman developed a technique for mapping viral genes
8.30 Hershey and Rotman developed a technique for mapping viral genes. [Photograph from G. S. Stent, Molecular Biology of Bacterial Viruses. © 1963 by W. H. Freeman and Company.]

34. RNA Virus
Retrovirus: RNA viruses that have been integrated into the host genome
Reverse Transcriptase: synthesizing DNA from RNA or DNA template
HIV and AIDS

35. 8.35a A retrovirus uses reverse transcription to incorporate its RNA into the host DNA. (a) Structure of a typical retrovirus. Two copies of the single-stranded RNA genome and the reverse transcriptase enzyme are shown enclosed within a protein capsid. The capsid is surrounded by a viral envelope that is studded with viral glycoproteins.

36. 8.35b A retrovirus uses reverse transcription to incorporate its RNA into the host DNA. (b) The retrovirus life cycle.