Bacterial Genetics Flashcards
3 ways of passing genetic information between bacterium
- Conjugation
- Transformation
- Transduction
Conjugation (general)
involves the transfer of DNA from a donor cell to a recipient cell through a sex pilus
Transformation (general)
involves the transfer of genetic information by naked extracellular DNA
Transduction (general)
involves the transfer of genetic information to a bacterium from a bacteriophage or between bacterial or yeast cells mediated by a phage vector
Why do we use an agar plate vs. a liquid growth medium?
agar plate allows us to tell the cells apart and quantify/isolate colonies
Minimal media
synthetic culture that contains only the material essential for the growth of a WT organism
Prototrophs
any strain of microorganism that can grow in solely minimal media
Auxotrophs
any strain of microorganism that differ from the WT strains and cannot grow on minimal media
Lederberg and Tatem experiment
two strains of bacteria that were each unable to synthesize DIFFERENT proteins were combined
together, the two auxotrophs were able to produce prototrophs
shows that genetic exchange occurred between the two strains (highly unlikely that this occurs by chance alone)
What direction does conjugation occur in?
unidirectional transfer of genetic material
from F+ (or HFr) to F-
Davis U-Experiment 1
Medium can pass back and forth between cells, but the cells cannot
No prototrophs were generated
Shows that direct contact is needed for conjugation to occur
F-pilus
hairlike tubular structure that allows for bacterial conjugation between 2 cells
Basic steps of conjugation
- F+ has a F-factor plasmid that encodes for the formation of the F-pilus
- Endonuclease enzyme separates the 2 strands of the F-factor plasmid
- One strand moves across the plasmid
- Separated plasmid strands are replicated in each cell
- Ligase closes the two circles
(now both cells have the ability to code for the F-pilus)
Wollman and Jacob
mixed HFr and F- strains and used interrupted mating technique to map genes by time
shows that genes were transferred at different points of entry and in different directions
proposed that DNA is circular
Difference between F+ cell and Hfr cell
F+ cell has an F-factor plasmid that can be passed
Hfr cell has F-factor plasmid integrated into the main chromosome as the point of origin
Why does Hfr not pass its F-factor?
Because it is embedded as the point of origin and would be the last thing to be passed between a cell
Why does (F+) + (F-) result in any recombination?
at low rates, F-factor plasmid will randomly integrate itself into the main chromosome
this results in some recombination taking place
creates rare, newly formed Hfr cells
Competent bacteria cells
the small number of bacteria cells that are able to express the correct protein receptor to take up extracellular “naked” DNA from the environment
Steps of transformation
- Competent bacteria cell takes up the extracellular DNA
- DNA strands separate
- Nucleases digest one of DNA strands
- Other strand pairs with host cell DNA and forms a heteroduplex
- after one round of cell division, a transformed and an untransformed cell are produced
Bacteriophages
viruses that use bacteria as their hosts
How do viruses bind to bacteria?
they have tail fibers that recognize specific receptors and bind to bacteria
2 cycles that viruses can trigger
- Lytic: kills the host
2. Lysogenic: postpones killing the host
Lytic cycle
Proteins are produced that shut down the host’s transcription and stimulate replication of the viral genome
Lysozyme enzyme is produced by a viral gene that breaks linkages between structural sugars in the bacterial cell wall
Cell wall breaks and new phages can infect neighboring bacteria cells
Lysogenic cycle
Prophage is inserted into the hosts main chromosome
Prophage is replicating with the normal bacterial chromosome
When the prophage is activated, the lytic cycle is activated which results in the host’s death
When does transduction occur?
occurs when the virus accidentally transfers some of another bacteria’s DNA when it infects a new cell
this does not happen on purpose
it is because of random cutting during replication
Zinder and Lederberg Experiment
used U-tube and observed prototrophs on one side
knew it was not transformation because they added DNAase to break down extracellular DNA
used a new filter with smaller pores that would block a virus from moving across the filter. they observed no prototrophs
indicates that transduction is occurring
