Microbial genetics Flashcards
What are 3 key characteristics of the bacterial genome?
- Typically a single circular chromosome
- No nuclear membrane, however the chromosome is restricted to defined region of the bacterial cell known as the nucleoid
- Small circular self replicating DNA molecules can be found in the cytosol (separate to the main chromosome) these are known as ‘Plasmids’
What is the ‘engine’ behind gene transfer in bacteria?
Spontaneous mutations ‘De novo’ as it provides new alleles
What was the first experiment done that proved horizontal gene transfer?
Strains of the streptococcus pneumoniae were found to have different effects on animals
- Smooth strain (had a polysaccharide capsule)
- Rough strain (no capsule)
How did Fred griffith’s experiment prove horizontal gene transfer?
- When the smooth strain was injected, the mouse died but the heat treated (dead) strain of the smooth did not kill the mouse.
- The rough strain doesn’t kill the mouse
- When injecting living R strain and dead s strain it was found that the mouse will die
- Therefore the dead s strain transferred its genes to the R strain
Why did the mouse die when given the S strain of streptococcus pneumoniae?
The capsule stops phagocytosis, so the smooth strain can multiple without being damaged and so can kill the organism via multiplication in the lungs
Why did the mouse not die when given only the R strain of streptococcus pneumoniae?
It has no capsule so will be destroyed via phagocytosis before it causes harm
What are the three types of horizontal gene transfer?
Transformation, Transduction and conjugation
How does transformation occur?
Involves uptake of short fragments of naked DNA by naturally transformable DNA
- Bacteria die and break apart and the chromosomal DNA falls around them
- The DNA can be chopped up via enzymes
- Pieces of DNA released by dead bacteria make its way into the cell of a live bacteria
- DNA is relatively heat-resistance (so can survive heat treatment)
How does transduction occur?
Involves transfer of DNA from one bacterium into another via bacteriophages
- Base plate will stick against cell wall and transfer its own DNA into the cell.
- Phage’s intention is to replicate itself by using the bacterias machinery
- In some cases, instead of putting own DNA inside a newly replicated head, a plasmid from the DNA is put in by mistake
- This does not affect phage’s normal function, so when the new phage connects on to a another specific bacteria, it will transfer the bacteria’s plasmid to the new bacteria instead of its own DNA.
How does conjugation occur?
Involves transfer of DNA material via sexual pilus (gene transfer tool) and requires cell to cell contact
- Sex pilus of one organism makes contact with another organism
- In most cases conjugation involves plasmids (not chromosomal DNA)
- The plasmids unravel and go through the tubes and reform on the other side
- Plasmids replicate all the time so bacterial that transfer don’t lose them
What is an example of transduction?
Cholera (infection of the small intestines)
- Not originally pathogenic
- The phage virus turns the bacteria into toxin producing bacteria
What is the lytic cell cycle?
When a bacteriophage injects its DNA and starts to reproduce, it will break out of the cell and release the reproduced phages (causes the bacteria to die)
What is the lysogenic cell cycle?
The phage DNA gets taken up by the bacterial chromosomal DNA - it becomes apart of the bacteria’s genome.
- This gets replicated via bacteria replication
- It increases itself without killing the bacteria
- May or may not ‘pop out’ and go through lytic cycle
