10 - Bacterial Genetics Flashcards
advantages of bacteria for genetics
- easily cultured
- short generation time
- haploid
E.coli
- double stranded circular DNA molecule
- DNa coils up in nucleoid
Bacteriophages of E.coli
viruses that infect bacteria
Bacteriophage life cycle
Lytic bacteriophage multiply and then lyse the bacterial cell releasing progeny bacteriophage particles.
Temperate/lysogenic bacteriophage can integrate into the bacterial chromosome and remain dormant, replicating along with the bacterial DNA
Describe the lytic cycle
- A phage attaches to a bacterial cell and injects it’s DNA into the cell
- All the DNA in the cell is hydrolysed
- The cell’s metabolic machienery js used to produce clones of the phage
- The phage then causes the bacterial cell to burst (usung lysozymes), releasing all the phages
Describe the temperate phage life cycle
- A phage will adhere to the bacterial cell and inject it’s DNA into the cell
- The phage DNA circularises
- The phage DNA integrates within the main bacterial chromosome, becoming a prophage
- The bacterial cell reproduces normally and copies the prophage DNA along with the bacterial DNA and transmits it to daughter cells
- Occasionally, the prophage may exit the bacterial chromosome, causing the cell to enter the lytic cycle
How can gene transfer and recombination take place
1 Transformation
2 Transduction
3 Conjugation
Transformation
the process by which bacteria will uptake of foreign DNA from the surrounding environment and incorporate it into its own genome
transduction
Transfer of bacterial genes from one bacteria to another by a bacteriophage
2 types of transduction
- generalized
2. specialized
generalised transduction
occurs only with virulent phage
Describe the process of generalised transduction
- A bacteriophage infects a cell, causing the cell to enter the lytic stage
- When the phages are being packaged, some of the hydrolysed bacterial DNA gets packaged into a phage
- The bacterial DNA will then be injected into another bacterial cell when it is infected by that phage
- Crossing over then occurs between the original bacterial DNA and the new host cell’s DNA, forming a recombinant
Specialised transduction
occurs only with temperate phage
Describe the process of specialised transduction
- A prophage within bacterial DNA exists the bacterial chromosome incorrectly, bringing some adjoining bacterial DNA with it
- The cell then enters the lyric phage and part of the bacterial DNA will be packaged into the phage capsid along with the phage DNA
- The phage will then infect a new host cell and insert its DNA
- crossing over occurs between the plasmid and the host cell’s chromosome, resulting in the dna from the original bacterial cell being incorporated into the new cell’s genome
conjugation
The ability to form sex pili and to transfer DNA by conjugation is determined by a plasmid called an F (for fertility) factor.
Describe the process of conjugation from an F+ cell to an F- cell
- An F+ forms a mating bridge with an F- cell
- The F+ cell then replicates it’s F plasmid and transfers it the F- negative cell
- The mating bridge then breaks and the new plasmid circularises, which results in the formation of a new F+ recombinant cell
Describe the process of conjugation between an Hfr cell to an F- cell
- In an Hfr cell, the F plasmid is incorporated into the main chromosome
- The Hfr cell then for,s a mating bridge with the F- cell
- The Hfr cell then replicates its DNA and starts transferring it to the F- cell
- The mating bridge breaks before all the chromosome goes through
- Crossing over occurs between the Hfr cell’s DNA and the F- chromosome result in recombinance. However the F- cell is still F- as it has not received the F gene
- The linear DNA is then dissolved by enzymes
At what level is bacterial gene expression regulated?
The transcription level
Describe the process of negative gene regulation with repressible enzymes, using tryptophan as an example
- When tryptophan (co repressor) is present, it binds to a repressor protein changing its shape
- The repressor protein can then bind to the operator (with in the promoter) and blocks transcription
- If tryptophan is not present, then the repressor protein can not bind to the promotor, allowing transcription to occur
Describe The process of negative gene regulation with inducible enzymes, using lactose as an example
- When lactose is absent, the repressor is active and can bind to the operator, preventing transcription
- When lactose is present, lactose binds to the repressor, this changes the shape of the repressor, causing it to become inactive
- The activator can no longer bind to the operator which means that transcription can go ahead
What is the difference between positive and negative gene regulation?
Positive regulation involves the binding of an activator to the operator, which turns on transcription.
Where as negative regulation involves the binding of a repressor protein which prevents transcription
Describe how cAMP causes positive gene regulation
- when glucose is scare, there is a high level of cAMP in the cell. The cAMP then binds to a cAMP receptor protein (CRP) and activates it. The activated protein then binds the CRP binding site and promotes transcription
- when glucose is abundant, there is a low level of cAMP in the cell. The CRP remains inactive so it does not bind to the CRP binding site. This prevents transcription.