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Bacterial Genetics Pin Lin (凌 斌), Ph.D.
Departg ment of Microbiology & Immunology, NCKU ext 5632 References: 1. Chapters 5 in Medical Microbiology (Murray, P. R. et al; 5th edition) 2. Chapter 25 in Biochemistry (Nelson, D. et al; 4th edition)
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Outline Introduction Replication of DNA Bacterial Transcription
Other Genetic Regulation (Mutation, Repair, & Recombination)
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Introduction Gene: a segment of DNA (or chromosome),
the fundamental unit of information in a cell Genome: the collection of total genes in an organism Chromosome: the large DNA molecule associated with proteins or other components
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Why do we study Bacterial Genetics?
Bacterial genetics is the foundation of the modern Genetic Engineering & Molecular Biology. The best way to conquer bacterial disease is to understand bacteria first.
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Bacterial vs Human Chromosome
E Coli: 1. Single circular chromosome, one copy (haploid) 2. Extrachromosomal genetic elements: Plasmids (autonomously self- replicating) Bacteriophages (bacterial viruses) 3. Maintained by polyamines, ex. spermine & spermidine Human: chromosomes, two copies (diploid) 2. Extrachromosomal genetic elements: Mitochondrial DNA Virus genome 3. Maintained by histones
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Replication of Bacterial DNA-I
Features: 1.Semi-conservative 2. Multiple growing forks 3. Bidirectional 4. Proofreading (DNA polymerase) Bacterial DNA is the storehouse of information. => Essential to replicate DNA correctly => Daughter cells
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Discovery of DNA synthesis
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Replication of Bacterial DNA-II
Replication of bacterial genome requires several enzymes: - Helicase, unwind DNA at the replication origin (OriC) - Primase, synthesize primers to start the process - DNA polymerase, synthesize a copy of DNA, first found by Arthur Kornberg - DNA ligase, link two DNA fragements - Topoisomerase, relieve the torsional strain during the process, found by James Wang
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Outline Bacterial Transcription Introduction Replication of DNA
Other Genetic Regulation (Mutation, Repair, & Recombination)
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Transcriptional Regulation in Bacteria
Bacteria regulate expression of a set of genes coordinately & quickly in response to environmental changes. Operon: the organization of a set of genes in a biochemical pathway. Transcription of the gene is regulated directly by RNA polymerase and “repressors” or “inducers” . The Ribosome bind to the mRNA while it is being transcribed from the DNA.
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Lactose Operon E Coli can use either Glucose or other sugars (ex: lactose) as the source of carbon & energy. In Glu-medium, the activity of the enzymes need to metabolize Lactose is very low. Switching to the Lac-medium, the Lac-metabolizing enzymes become increased for this change . These enzymes encoded by Lac operon: Z gene => b-galactosidase => split disaccharide Lac into monosaccharide Glu & Gal Y gene => lactose permease => pumping Lac into the cell A gene => Acetylase
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Lactose Operon-Negative Control
Lac Operon: - Lac metabolism - Under pos & neg control Negative ctrl - Repressor - Inducer (Allolactose) - Operator In presence of Lactose
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Lactose Operon-Positive Control
Activator: CAP-cAMP (catabolite gene-activator protein) CAP RNA pol In absence of Lactose
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Tryptophan Operon Negative control - Repressor
- Corepressor (Tryptophan) - Operator
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Transcription termination signal
Attenuation Couple Translation w/ Transcription Sequence 3:4 pair G-C rich stem loop Called attenuator Like transcriptional terminator Sequence2: 3 pair - weak loop won’t block translation Transcription termination signal
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Outline Introduction Replication of DNA Bacterial Transcription
Other Genetic Regulation (Mutation, Repair, & Recombination)
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Types of mutations 1. Base substitutions
Silent mutation – No change of amino acid Missense mutation – Switch to another amino acid Nonsense mutation – Change to a stop codon 2. Deletion & Insertion - Change more base pairs in DNA => frameshift => truncated gene product 3. Rearrangements - Duplication, Inversion, Transposition
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Induced mutations Physical mutagens: e.g., UV irradiation
(heat, ionizing radiation) Chemical mutagens Base analog Frameshift intercalating agents Base modification Transposable elements
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DNA Repair 1. Direct DNA repair (e.g., photoreactivation)
2. Excision repair Base excision repair Nucleotide excision repair 3. Post-replication or Recombinational repair 4. SOS response: induce many genes 5. Error-prone repair: fill in gaps with random sequences Thymine-thymine dimer formed by UV radiation
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Excision repair Base excision repair Nucleotide excision repair
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Double-strand break repair
(postreplication repair)
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SOS repair in bacteria Inducible system used only when error-free mechanisms of repair cannot cope with damage Insert random nucleotides in place of the damaged ones Error-prone
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Mechanisms of gene transfer
Transformation: uptake of naked exogenous DNA by living cells. Conjugation: mediated by self-transmissible plasmids. Transduction: phage-mediated genetic recombination. Transposons: DNA sequences that move within the same or between two DNA molecules
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Importance of gene transfer to bacteria
Gene transfer => a source of genetic variation => alters the genotype of bacteria. The new genetic information acquired allows the bacteria to adapt to changing environmental conditions through natural selection. Drug resistance (R plasmids) Pathogenicity (bacterial virulence) Transposons greatly expand the opportunity for gene movement.
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Transformation Artificial transformation Natural transformation
(conventional method and electroporation) Natural transformation
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Avery, MacLeod, and McCarty (1944)
Demonstration of transformation Avery, MacLeod, and McCarty (1944)
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Gene exchange by Plasmids
Extrachromosomal Autonomously replicating Circular or linear (rarely) May encode drug resistance or toxins Various copy numbers Some are self-transmissible
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Conjugation mediated by self-transmissible plasmids
(e.g., F plasmid; R plasmids)
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F plasmid --an episome F plasmid
F plasmid can integrate into bacterial chromosome to generate Hfr (high frequency of recombination) donors Hfr strain Excision of F plasmid can produce a recombinant F plasmid (F’) which contains a fragment of bacterial chromosomal DNA F’ plasmid
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phage-mediated genetic recombination
Transduction phage-mediated genetic recombination Generalized v.s. specialized transduction
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Transposons Mobile genetic elements May carry drug resistance genes
Sometimes insert into genes and inactivate them (insertional mutation) Transposons
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Spread of transposon throughout a bacterial population
Trans-Gram gene transfer
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Mechanisms of evolution of Vancomycin-resistant Staphylococcus Aureus
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Cloning Cloning vectors plasmids phages Restriction enzymes Ligase
In vitro phage packaging
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Library construction Genomic library cDNA library
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Applications of genetic engineering
1. Construction of industrially important bacteria 2. Genetic engineering of plants and animals 3. Production of useful proteins (e.g. insulin, interferon, etc.) in bacteria, yeasts, insect and mammalian cells 4. Recombinant vaccines (e.g. HBsAg)
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Take-Home Question: Mutations are good or bad for bacteria
The End & Thank You
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Mechanism of Recombination
Homologous recombination Site-specific recombination Transposition Illegitimate recombination Intermolecular Intramolecular Double crossover Homologous recombination
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E Conjugational transposon
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Spread of transposon throughout a bacterial population
Trans-Gram gene transfer
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Cloning Cloning vectors plasmids phages Restriction enzymes Ligase
In vitro phage packaging
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Library construction Genomic library cDNA library
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Applications of genetic engineering
Construction of industrially important bacteria Genetic engineering of plants and animals Production of useful proteins (e.g. insulin, interferon, etc.) in bacteria, yeasts, insect and mammalian cells Recombinant vaccines (e.g. HBsAg)
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Bacteriophage (bacterial virus)
Structure and genetic materials of phages Coat (Capsid) Nucleic acid Icosahedral tailess Icosahedral tailed Filamentous
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Life cycle Phage l as an example Lytic phase Lysogenic phase
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Virulent phages: undergo only lytic cycle Temperate phages: undergo both lytic and lysogenic cycles Plaques: a hollow formed on a bacterial lawn resulting from infection of the bacterial cells by phages.
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