Bacterial genetics Flashcards
What are 3 differences between prokaryotic and eukaryotic genetics?
Prokaryotes have extra-chromosomal plasmids whereas eukaryotes don’t
Prokaryotes chromosomes are located in the cytoplasm whereas eukaryotes are located in the nucleus
Prokaryotes have coupled transcription and translation, whereas eukaryotes have separate transcription and translation
How do bacteria pass on genetic information?
Horizontal gene transfer enables genetic variation. Vertical gene transfer takes place in binary fission.
Why is there genetic variation in bacteria?
Respond to new selective pressures
To survive adverse environmental conditions
To exploit new environments that it encounters
What are genetic changes passed through VGT?
Spontaneous mutations
Single-nucleotide polymorphisms (SNPs)
e.g. CAT to CGT
Mutation rates are low; slow – driven by replication rate
Selects out mutants – M. tuberculosis; point mutation in rpoB gene
What are the three processes of HGT?
Conjugation - transfer of plasmids via pili
Transduction- transfer of DNA via bacteriophage (virus)
Transformation - uptake of DNA from the environment
What are properties of bacterial plasmids?
Extra-chromosomal pieces of DNA
Small circular molecules
Self-replicating
Can be present in multiple copies
Can have genes required for:
Virulence
Antibiotic resistance
Some toxins
Genes that promote adherence to host cells
What is involved in conjugation?
DNA polymerase brings bacteria together and a donor bacteria passes a plasmid to a recipient.
What is involved in transduction?
Phage infects bacterium -> circularizes ->either lysogeny or lysis
Phage DNA integrates and bacterium grows as lysogen
Phage DNA replicates -> new phage particles are produced and lyse bacterium -> release of phage
How are transductants formed?
Bacterial DNA is partially degraded by phage -> a phage can take up bacterial DNA -> recipient strain infected -> recombinant recipient transductant
What are pathogenicity islands (PAIs)?
Transductant formation can result in PAIs which are large pieces of DNA that encode on or more virulence factors. They are only present in genomes of pathogenic strains.
How does transformation occur?
Can occur naturally or be artificially induced; conditions must be favourable and bacterial cells mist be competent
What is the importance of HGT for bacterial pathogens?
Gene pool is very large allowing for rapid emergence of bacterial pathogens; a single species can colonise diverse niches and cause a range of disease
Diversity allows escape from antibiotics or vaccinations
What are some examples of HGT?
Plasmid-encoded heat-labile enterotoxin of E. coli
PAI encoding haemolysin, pili and cytotoxins
Phage encoded dtxR gene from C. diphtheriae
Transfer of antibiotic resistance in Gram -ve bacteria: beta-lactamases plasmid encoded; beta-lactamases chromosomally encoded
