The Prokaryotic Chromosome

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The Prokaryotic Chromosome

• Genetic Analysis of Bacteria

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Archaea
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Bacteria Key Genetic Features

• Large numbers
• Small
• Fast growth of colony that are genetically
  identical
• Examine rare mutation events

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Circular DNA 4 to 5 Mbp
Divides by binary fission Asexual
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Finding Mutations in Bacterial Genes

• Colony morphology shiny, dull
• Resistance to antibiotics or phages
• Auxotrophs cant grow on minimal medium
• Ability of cells to breakdown compounds
• Essential genes that are temperature sensitive

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Techniques used to study Bacterial Genetics

• Replica Plating
• Treatments with Mutagens
• Enrichment of mutant cells
• at the expense of cells

4. Visible mutant phenotypes
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Bacterial colonies producing GFP-like proteins
cloned from red (left) and green (right) morphs
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Structure and Organization of Bacterial
Chromosome
Poster Child of Bacteria
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E. coli

• 4.6 million base pairs
• 90 encode proteins
• Compare to 5 for eukaryotes
• Over 4000 genes
• 40 unknown function -Open reading frames (ORFs)
• Remnants of bacteriophage genomes

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DNA Replication in E. coli
Occurs at oriC Ends at terC
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Plasmids carry genes not usually needed
But may be beneficial
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Gene Transfer in Bacteria

• Transformation
• Conjugation
• Transduction

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Bacterial Conjugation
Caught in the Act
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Gene Transfer
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Integration of F plasmid
Hfr Bacterium
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Artificial Transformation
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A useful plasmid in genetics
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Example of plasmid used in genetic studies
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Mapping genes by interrupting Mating
Gene order established
a
Genes a, b, c, etc expressed in
recipients depends on interruption times
a, b, c
a, b, c, d
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Start with 3 cultures of bacteria strain A
(thr-leu-) alone strain B (bio-met-thi-)
alone a mixture of strain A and strain B
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Grow all 3 cultures in minimal media
supplemented with the 5 nutrients (threonine,
leucine, biotin, methionine and
thiamine) Collect the cells, wash them in
minimal media, and plate them on minimal media
plates without any supplements
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The only cells which can grow are cells that no
longer have any nutrient requirements.
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Basic Conjugation Experiment
RESULTS
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Could cross-feeding have been responsible?
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Direct contact is needed
No Growth
No Growth
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GENERALIZED TRANSDUCTION
Transfer of bacterial genes between related
strains of bacteria
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Isolation of mutants top priority in
bacterial genetics
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Transposon insertion Inactivates gene
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How Bacteria Cause Disease
Steps of Infection Natural and Synthetic
Defenses Bacterial Evasion and Drug
Resistance Genetic basis of Virulence
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Finding the genes contributing to virulence
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