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Immunology Pt. 2 > Bacterial Genetics > Flashcards

Bacterial Genetics Flashcards

1
Q

Differences between bacteria and Eukaryotes Chromosomes:

  • Compartmentalization:
  • Plasmids:
  • DNA:
A

Chromosomes: Bacteria have single chromosome that is circular < 5mb; Several to many linear chromosomes > 5mb

  • Compart: Bacterial genome not compartmentalized
  • Plasmids: Eukaryotes have no plasmids
  • DNA: Chormosome condensed by supercoiling in bacteria; in Eukaryotes wrap around histones
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2
Q

Mechanism of replication in bacteria

A

Single origin, second round of replication can start before cell divides

Replicated DNA partitioned into daughter cell - binary fission

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3
Q

Differences in transcription/translation

A

Bacteria - Genes encoded with operons; no introns or exons; 1 RNA polymerase

Eukaryotes - No operons; introns and exons; 3 RNA polymerase

* in bacteria, transcription/translation coupled since no compartmentalization

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4
Q

How do bacteria generate diversity

A
  1. Replication errors - introduce mutations into gene
  2. Exchanging genetic material
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5
Q

How do replication errors occur in bacteria? What is the frequency?

A
  • DNA polymerase misincorporates nucleotide and does not correct this mistake through proofreading activity
  • Frequency is 1 mutation/300 chromosome replications = 10^(-6) mutations per genome per generation
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6
Q

Horizontal Gene transfer (vs. vertical transmission in Eukaryotes)

A

There is no natural species boundary in prokaryotes like there is for Eukaryotes

  • Bacteria can acquire genetic material from another organism
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7
Q

Why/How does bacteria perform gene exchange?

A
  • The species barrier in bacteria is less stringent than in Eukaryotes
  • Only need a single organism in the population to survive
  • Help outcompete or kill other organisms
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8
Q

Exchangeable genetic elements

A
  1. Plasmids
  2. Insertion sequences/Transposons
  3. Pathogenicity islands
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9
Q

Of the exchangeable genetic elements, most concern involves genes encoding _____ ______ and _____ _______ ______

A
  • Virulence factors
  • Antibiotic resistance determinants
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10
Q

Methods of exchange

A
  1. Transformation
  2. Transduction
  3. Conjugation
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11
Q

Plasmids

Size:

Exchange:

A

DNA molecules that replicate independently of bacterial chromosomes

Size: from 1500bp to 400,000bp

Exchange: transformation, conjugation, transductoin

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12
Q

Single copy plasmids

A

F plasmids or episomes

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13
Q

Insertion sequences/ transposons

A
  • Mobile genetic elements
  • linear DNA segment - often disrupt genes in recipient bacterium following transfer
  • Cannot replicate on their own - often transferred on plasmid or by bacteriophage
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14
Q

ITR

A

Inverted terminal repeats at the ends of insertion sequences and simple transposons

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15
Q

Transposase

A

Enzyme that recognizes ITR and cuts the DNA allowing transposition of element from one location to another

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16
Q

Difference between insertion sequences and composite transposons

A

Insertion sequence: Sequence with inverted repeats on each side

Composite transposons: Insertion sequence on each end with central region: ex. genes for antibiotic resistance - all transposed together

17
Q

Phase variation

A

variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial population

18
Q

Phase variation in Escherichia coli that are responsible for UTIs (UPEC)

A
  • Associated with site-specific inversion of an insertion sequence
  • Spontaneous switch between non-fimbriated and fimbriated forms
  • Fimbriae promote attachment to urinary tract epithelial cell surface
19
Q

Genomic pathogenicity islands

A

Segment of bacterial genome = carried on plasmid or by bacteriophage

20
Q

Distinguishing Genomic Pathogenicity Islands

A
  • G+C content differs from majority of chromosome
  • Can encode adherence factors, invasion genes, toxins etc.
  • Vary in size from 10,000bp to 200,000bp
  • Cannot replicate by iteself
21
Q

Transformation:

Conjugation:

Transduction:

A

Transformation: DNA taken up from environment

Conjugation: Direct cell-to-cell contact

Transduction: Mediated by bacteriophage

22
Q

Bacterial transformation

A
  • Uptake of DNA from environment
  • Occurs in gram + and gram - bateria
  • DNA released following lysis of bacteria
  • dsDNA bound then processed to ssDNA before internalization
  • Homologous recombination
23
Q

Conjugation

A
  • DNA passed unidirectionally from Donor to Recipient
  • Requires physical contact
  • Both gram + and gram -
  • DNA transferred through mating bridge
  • Transfers plasmids, transposons, and chromosomal DNA
  • Donor retains copy of original genetic material
24
Q

Mechanism of conjugation

A
  1. Generation of pilus
  2. Pilus retraction to form mating bridge
25
Q

Transduction-transfer of DNA via bacteriophage (types of bacteriophage)

A
  • Bacteriophages are obligate intracellular parasites
  • 3 main types: icosahedral head; icosahedral head with tail; filamentous
  • Single nucleic acid molecule
  • Nucleic acid protected by protein coat or capsid
26
Q

Lytic vs Lysogenic lifecycle

A

Lytic: Make new progeny phage = phage replicated independently of host genome

Lysogenic: Vertical transmission of phage = phage genome integrated and replicated with bacterial genome

* temperate phage can switch between these lifecycles

27
Q

Mechanism of transduction

A
  • During lytic replication, fragment of bacterial genome packaged into phage particle rather than phage genome = transducing phage
  • Upon infection of new bacterium, transducing phage releases fragment of bacterial genome into host
  • Recombination between introduced DNA and host DNA result in stable inheritance
28
Q

Cholera toxin phage (Ctx)

A
  • Filamentous bacteriophage that infects Vibrio cholera
  • Comprised of core element and repetitive sequence
  • Ctx is 5B to 1a (5 binding subunits with 1 enzymatic subunit)
  • Phage morphogenesis and enterotoxic activity
29
Q

Mechanism of cholera toxin activity

A
  • A-5B toxin → B subunit binds ganglioside GM1
    • A subunit internalized and interacts with G proteins regulating adenylate cyclase
  • Induces conversion of ATP to cAMP which results in enhanced secretion of water and electrolytes (diarhhea)
30
Q

Shiga Toxin phage (Stx)

A
  • Icosahedral head with short tail
  • Temperate bacteriophage: lytic and lysogenic
    • Shiga toxin is made by both Shigella dysenteriae and EHEC strains of E. coli
  • Induces severe diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome (HUS)
31
Q

Mechanisms of Shiga Toxin

A
  • A-5B toxin
  • Mechanism identical between Shiga- and Shiga-like toxin
  • B subunit binds Gb3 glycolipid
  • A subunit is translocated into cytosol and modifies ribosome acceptor site
  • Blocks protein synthesis

Immunology Pt. 2 (11 decks)

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