L19: Microbial Genetics Flashcards
2 types of bacterial genetic material
- Chromosome – large circular DNA molecule
2. Plasmid – small circular DNA molecule that is transferred to daughter cells
Episomes
Plasmids that can insert into the bacterial chromosome
Homologous recombination
Exchange of DNA between two DNA molecules based on homology (have to be somewhat similar in sequence) – RecA binds and mediates this between linear DNA and chromosome
Why is homologous recombination in bacteria important?
For horizontal gene transfer (transformation, conjugation, transduction)
Transformation
Uptake of free DNA from the environment to allow bacteria to gain new traits quickly (homologous recombination happens when DNA is released into the environment and imported into cell)
Conjugation
DNA transfer directly from one bacterial cell to another through cell contact to acquire new DNA from another bacterium; F-factor is important
F-factor (fertility factor)
Carries genes in tra operon that encode components of the sex pilus and also oriT
Bacterial mating types for conjugation
F- = no F factor (always recipient)
F+ = contains F factor
Hfr (high-frequency recombination) = F factor integrated into bacterial chromosome
F+ x F-
Unidirectional transfer where F- undergoes sex change to F+; only single strand is transferred (like an arrow being shot)
Hfr x F-
- -Sex pilus bridge forms
- -DNA is transferred from oriT in (including chromosomal DNA) – tra genes come in last
- -Normally sex change does not occur (remains F-, just has new DNA)
Transduction
Transfer of bacterial DNA via a bacteriophage (virulent/lytic phages or temperate phages)
Process of generalized transduction
- -Done with a lytic phage
- -Phage DNA is injected into bacteria
- -Phage produces enzymes that degrade bacterial chromosome
- -When progeny are packed up, some bacterial DNA gets included in phage head
- -When phage infects another bacteria, DNA can be incorporated into its chromosome via homologous recombination
Process of specialized transduction
- -Done with lysogenic phage
- -Phase incorporates into chromosome as prophage
- -Conversion to lytic life cycle occurs
- -Upon excision from the genome, a small portion of chromosome adjacent to where phage was integrated is packaged into phage head
Importance of lysogenic phages
Many prophages are important in virulence of certain bacteria
Only method of DNA transfer in bacteria that doesn’t require homologous recombination
F+ x F- conjugation
3 categories of antibiotic resistance
- Intrinsic
- Chromosome-mediated
- Plasmid-mediated
Intrinsic antibiotic resistance
Not transferable between bacteria, not increasing among bacterial populations
Chromosome-mediated antibiotic resistance
Chromosomal genes encoding antibiotic resistance that arise through random mutation or acquisition of DNA through horizontal gene transfer and incorporation into genome via homologous recombination
Plasmid-mediated antibiotic resistance
Genes on a plasmid that encode antibiotic resistance; have resistance factors (R-factor) that are made up of a resistance transfer factor and a resistance determinant
Transposons
Mobile DNA elements that can transfer themselves or a copy from one molecule of DNA to another; contain indirect repeat sequences on each end so they can jump around
Why are transposons dangerous for antibiotic resistance?
Multi-drug resistant plasmids can be constructed and passed between bacterial species due to transposons
Integrons
Diverse group of genetic elements that encode a site-specific recombination system that can capture gene cassettes (often antibiotic resistant)
Resistance gene cassettes
New genes that are expressed in coordinated manner under control of integron promoter
How do bacteria regulate gene expression at different levels?
RNA polymerase binds to a promoter to promote transcription; repressors can inhibit transcription in negative gene regulation and activators can begin transcription in positive gene regulation
