Bacterial Genetics Flashcards
Differences between bacteria and Eukaryotes Chromosomes:
- Compartmentalization:
- Plasmids:
- DNA:
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
Mechanism of replication in bacteria
Single origin, second round of replication can start before cell divides
Replicated DNA partitioned into daughter cell - binary fission
Differences in transcription/translation
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
How do bacteria generate diversity
- Replication errors - introduce mutations into gene
- Exchanging genetic material
How do replication errors occur in bacteria? What is the frequency?
- 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
Horizontal Gene transfer (vs. vertical transmission in Eukaryotes)
There is no natural species boundary in prokaryotes like there is for Eukaryotes
- Bacteria can acquire genetic material from another organism
Why/How does bacteria perform gene exchange?
- 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
Exchangeable genetic elements
- Plasmids
- Insertion sequences/Transposons
- Pathogenicity islands
Of the exchangeable genetic elements, most concern involves genes encoding _____ ______ and _____ _______ ______
- Virulence factors
- Antibiotic resistance determinants
Methods of exchange
- Transformation
- Transduction
- Conjugation
Plasmids
Size:
Exchange:
DNA molecules that replicate independently of bacterial chromosomes
Size: from 1500bp to 400,000bp
Exchange: transformation, conjugation, transductoin
Single copy plasmids
F plasmids or episomes
Insertion sequences/ transposons
- 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
ITR
Inverted terminal repeats at the ends of insertion sequences and simple transposons
Transposase
Enzyme that recognizes ITR and cuts the DNA allowing transposition of element from one location to another
Difference between insertion sequences and composite transposons
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
Phase variation
variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial population
Phase variation in Escherichia coli that are responsible for UTIs (UPEC)
- 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
Genomic pathogenicity islands
Segment of bacterial genome = carried on plasmid or by bacteriophage
Distinguishing Genomic Pathogenicity Islands
- 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
Transformation:
Conjugation:
Transduction:
Transformation: DNA taken up from environment
Conjugation: Direct cell-to-cell contact
Transduction: Mediated by bacteriophage
Bacterial transformation
- 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
Conjugation
- 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
Mechanism of conjugation
- Generation of pilus
- Pilus retraction to form mating bridge
Transduction-transfer of DNA via bacteriophage (types of bacteriophage)
- 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
Lytic vs Lysogenic lifecycle
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
Mechanism of transduction
- 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
Cholera toxin phage (Ctx)
- 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
Mechanism of cholera toxin activity
- 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)
Shiga Toxin phage (Stx)
- 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)
Mechanisms of Shiga Toxin
- 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
