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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
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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
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• 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
The Bacterial Genome
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• Extra chromosome, small circular DNA
• Episomes – plasmids / F (fertility) factor
The Bacterial Genome
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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
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• Conjugation:
• Direct transfer via connection tube, one-way traffic from donor cells to recipient cells.
• It is not a reciprocal exchange of genetic information.
Gene Transfer in Bacteria
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• Conjugation:
• F+ cells donor cells containing F factor
• F− cells recipient cells lacking F factor
• Sex Pilus – connection tube
Gene Transfer in Bacteria
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• 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
Gene Transfer in Bacteria
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• 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.
Gene Transfer in Bacteria
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• Conjugation:
• Natural Gene Transfer and Antibiotic Resistance
• Antibiotic resistance comes from the actions of genes located on R plasmids that can be transferred naturally.
Gene Transfer in Bacteria
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• 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.
Gene Transfer in Bacteria
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• Transformation:
• A bacterium takes up DNA from the medium.
• Recombination takes place between introduced genes and the bacterial chromosome.
Gene Transfer in Bacteria
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• Transformation:
• Competent cells: cells that take up DNA
• Transformants: cells that receive genetic material
• Cotransformed: cells that are transformed by two or more genes
Gene Transfer in Bacteria
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• 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
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• Transduction:
• Bacterial viruses (bacteriophage) carry DNA from one bacterium to another.
• Transduction usually occurs between bacteria of the same or closely related species.
Gene Transfer in Bacteria
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• 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.
8.2 Viruses Are Simple Replicating Systems Amenable To Genetic Analysis
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• Transduction: using phage to map bacterial genes
• General transduction: any genes can be transferred
• Specialized transduction: only a few genes can be transferred
Techniques for the study of bacteriophages
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• Gene Mapping in Phages
Techniques for the study of bacteriophages
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• Retrovirus: RNA viruses that have been integrated into the host genome
• Reverse Transcriptase: synthesizing DNA from RNA or DNA template
• HIV and AIDS
RNA Virus
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