Chapter4 Heredity and variation of bacteria

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Chapter4Heredity and variation of bacteria
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outline

• Concepts
• Genetic materials of bacteria
• Mechanism of variation

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concepts

• Heredity
• Variation
• Genotypic variation
• Phenotypic variation

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Variation of bacteria

• Variation in morphology and structure
• lose capsule (Pneumococcus)
• H O
• Lose Spore
• L-form

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Variation

• Variation in virulence
• Bovine TB BCG
• 13years, 230 generation
• C. diphtheriae
• b-corynephage, diphtheriae toxin

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Variation

• Variation in drug resistance
• Penicillin resistant strains of staphylococcus
  aureus
• Variation in colony
• S-R
• Variation in enzyme activity
• Phenotypic
• Genotypic

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I Genetic materials of bacteria

1. Chromosomes
2. Plasmids
3. Phage
4. Transposable elements

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1 Chromosomes
DNA forms ds-DNA, circle
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• Size
• E.coli 1300mm, 4288gene
• Rolling-circle pattern of replication

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2 Plasmids

• Extrachromosomal genetic elements that are
  capable of autonomous replication.
• Small double-stranded DNA molecules, usually
  circular
• exist independently of host chromosome
• autonomously replicating (replicon)
• may disappear spontaneously or by induction (UV)
• incompatibility and compatibility

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Classification of Plasmids

• Transfer properties
• Conjugative
• 40-100kbp
• eg. F, R plasmid
• Nonconjugative
• lt15kbp, transfer by mobilization
• eg. ColE1 plasmid

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• Phenotypic effects
• Fertility plasmid,F plasmid
• coding sex pilus

Resistance plasmid, R plasmid resistance
transfer factor resistance determinant
Virulence plasmid Coliciogenic plismid
R determinant
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3 Bacteriophage, phage
T4 bacteriophages infecting E.coli.
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Bacteriophage (Phage)

• Definition
• Bacterial virus.
• Virus of bacteria, fungi, actinomyces, and
  spirochete.

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Composition and Structure

• Composition
• Nucleic acid
• DNA or RNA
• Protein
• Protection
• Infection

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• Structure (T4)
• Head or capsid
• Tail

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Infection of Host Cells

• Attachment
• Sheath contraction
• Nucleic acid injection

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Attachment and injection
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Types of Bacteriophage

• Lytic or virulent phage
• (e.g., T4)
• Lysogenic or temperate phage
• (e.g., l)

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Virulent Phage

• Lytic or virulent phage
• Phage that can only multiply within bacteria and
  kill the cell by lysis. (e.g., T4)

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Lytic Phage Multiplication Cycle
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Lytic cycle

• Attachment
• Injection and uncoating
• Biosynthesis
• Eclipse
• Early proteins
• Phage DNA synthesis
• Late proteins
• Intracellular accumulation
• Maturation and releasing

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Lysogenic Phage

• Lysogenic or temperate phage
• Phage that can either multiply via the lytic
  cycle or enter a quiescent state in the bacterial
  cell. (e.g., l)
• Prophage
• Lysogen,Lysogenic bacterium
• Lysogenic conversion

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Lysogenic phage /temperate phage
Lysogenic bacterium /lysogen
prophage
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• Lysogenic or phage conversion
• Definition A change in the phenotype of a
  bacterial cell as a consequence of lysogeny
• Modification of Salmonella O antigen
• Toxin production by Corynebacterium diphtheriae

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Lytic versus lysogenic infection by phage Lambda
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4 Thransposable element

• Concept
• Segments of DNA that are able to move around the
  genome.
• Properties
• Random movement
• Jumping genes or movable genes
• First discovered in the 1940s by Barbara
  McClintock during her study on maize
  genetics.(won the Nobel prize in 1983)

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Types of Transposon

• Insertion sequences (IS)
• A short sequence of DNA containing only the genes
  for those enzymes required for its transposition.

• Importance
• Mutation
• Plasmid insertion

IR inverted repeat
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Types of Transposon

• Transposons (Tn) or complex Tn
• contain genes other than those required for
  transposition (eg. Antibiotic resistance or toxin
  genes)

• Importance
• Antibiotic resistance

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II mechanism of variation

1. Mutation
2. Gene transfer and recombination

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i Mutations in bacteria

• Changes in DNA sequences
• Base substitutions, deletions, insertions,
  rearrangements
• Spontaneous mutation 10-8-10-6
• Backword mutation or reverse mutation

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ii Gene transfer and recombination

1. Transformation
2. Transduction
3. Conjugation
4. Lysogenic conversion
5. protoplast fusion

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General Features of Gene Transfer in Bacteria

• Unidirectional
• Donor to recipient
• Donor does not give an entire chromosome
• Gene variation can occur between species

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1 Transformation

• Definition Gene transfer resulting from the
  uptake of DNA from a donor.
• Competence of the recipient (Bacillus,
  Haemophilus, Neisseria, Streptococcus)

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Griffiths transformation experiments
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Bacterial Transformation With DNA Fragments
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2 Conjugation

• Definition Donor DNA transferred to recipient
  cell through sex pilus.

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F plasmid
Mechanism of F x F- Crosses Pair formation
Conjugation bridge DNA transfer Origin of
transfer Rolling circle replication
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Physiological States F plasmid
F plasmid
Hfr, high frequency of recombination
F plasmid
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Mechanism of Hfr x F- Crosses
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Mechanism of F x F- Crosses
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Resistance plasmid, R plasmid

• RTF (resistance transfer factor)
• transfer genes
• code pili
• R determinant (resistance determination )
• resistance genes
• transponsons

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3 Transduction

• Definition Gene transfer from a donor to a
  recipient by way of a bacteriophage

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Types of transduction

• Generalized - Transduction in which potentially
  any donor bacterial gene can be transferred
• Specialized (restricted) - Transduction in which
  only certain donor genes can be transferred

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Generalized transduction by bacteriophages
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Generalized Transduction

• Infection of Donor
• Phage replication and degradation of host DNA
• Assembly of phages particles
• Release of phage
• Infection of recipient
• Legitimate recombination

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specialized transduction by a temprerate
bacteriophage
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specialized transduction
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• lysogenic conversion
• DNA recombination
• protoplast fusion

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Practical implications

• Application in diagnosis, treatment and
  prevention of infectious diseases
• Eg. L-form
• PCR
• Detection of mutagenicity
• Application in genetic engineering

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The Ames Test for mutagenicity