11.Horizontal genetic transfer in bacteria Flashcards
What are characteristic of horizontal genetic transfer
-It’s only in one direction-from donor to the recipient
-only part of a chromosome is transferred in case of foreign DNS
what’s exception to the rule that only a part of the chromosome is transferred in case of foreign DNA
Merozygotes (partial diploidy)
Can plasmids transfer and replicate autonomously
Yes
Can genetic transfer occur between different species
Yes
What types of bacterial horizontal genetic transfer do we have
Transformation
Conjugation
Transduction
What is transformation
Introduction of foreign DNA from the extracellular environment into the bacterial cell which is maintained in a heritable form
When does transformation occur
In a certain stage of growth and it’s finely regulated
What’s competent bacterial cell
Bacterial cell that’s able to uptake DNA from the extracellular environment
When is competence acquired
In a particular stage of the growth curve and under particular nutritional conditions
Where is transformation used in research
In artificial introduction of plasmids into bacterial cell
What kinds of artificial transformations do we have
-Chemical: By CaCl2(Escherichia coli and other Gram -); It’s partial permeabilization in ice and then heat shocked. As the result, dsDNA enters the cell
-Physical: it can be used in all cells; uses process called electroporation which enhances permeability using electric field. Electric shock generates transient spores into the membrane. As the result, dsDNA enters the cell
Where do clones of DNA of interest in transformation grow
On selective medium
What is conjugatioon
transfer of genetic material that requires physical contact between the donor and recipients cell. Transfer of plasmids or chromosome portions
What is exchanged in conjugation
Conjugation factors
What is system that allows physical exchange, in +conjugation*, encoded by
Conjugative plasmids or transposones
What other factors can conjugative plasmids carry except conjugation factors
Genes for antibiotic resistance, toxins, colicines, complete metabolic pathway, etc.
Cam conjugation occur between 2 species of different genera(genus)
Yes
What are steps of conjugation
F+ donor containing F plasmid is capable of synthesizing sex pili
1.Contact: Sex pilus contacts the recipient F- cell
2. Activation of DNA transfer: plasmid is activated for transfer when endonuclease cleaves one strand of DNA at origin of transfer
3. Plasmid transfer: Sex pilus retracts and pulls donor and recipient cells together. F plasmid is transferred as single stranded DNA molecules
4. Synthesis of functional plasmid: Complementary strands to both F plasmid strands are synthesized in the donor and recipient cells. Both cells are F+ and synthesize sex pilus
What is F factor in conjugation
Conjugative plasmid that is able to integrate into the host chromosome or to exist in episomal form. It encodes for all the genes necessary for the conjugation process
In what forms can F factor exist
3:
-Free plasmid(episomal form)-independently from the host genome
-Plasmid integrated in the host genome
-F’ plasmid, with part of the host chromosome integrated into the plasmid
What’s transduction
Transfer of genetic material mediated by viruses
What kind of transduction do we have
Generalized and Specialized
What’s generalized transduction
Foreign DNA fragments are transferred as parts of the phage genome.
What’s specialized transduction
Temperate phages are vectors of foreign DNA fragments. A portion of host genome is improperly integrated into the phage DNA because the viral DNA was integrated in the host chromosome
What type of nucleic acid do virion contain
Only one, DNA or RNA
What’s some important characteristic of viruses
-Do not increase in size or replicate by binary fission
-Viral genome does not encode machinery for the production of energy or metabolism
-Viral genome does not encode machinery for translation
What are some basic T4v phage components
Head(which contains DNA),
Neck and collar,
Core,
Sheath,
End plate,
Fibers
How do viruses infect bacteria
By attachment and inoculation into the cytoplasm
What happens to host functions because of the virus
They are blocked
What kind of cycle do we have in phages
Lytic and lysogenic
Can phages carry toxic genes
Yes
What’s lytic cycle
Virulent phages. Often called reproductive cycle. Virus enters the cell and virus is repeatedly replicated until the cell lyses, releasing phages
What’s lysogenic cycle
Temperate phages. It doesn’t kill bacteria. It creates many copies of viral DNA
What does transducing phages have
Their particles contain host genes from donor cell
What can phages made by specialized transduction do
They are defective and can transduce the genes for galactose catabolism or the production of a toxin
What are some results when transferring bacteria genetic material
- Degradation by recipient’s enzymes (usually)
- Autonomous replication(only if it possesses origin of replication)
- Recombination with recipient chromosome and integration
- CRISPR/Cas degradation of donor DNA as an antiviral response mechanism
What is recombination
Physical exchange of DNA between two genetic elements that involves breaks and re-ligation of DNA during the rearrangement
What kind of recombination is there
Homologous and site-specific
When does homologous recombination happen
HR occurs when there are long regions of similar nucleotide sequence
What the process of homologous recombination
- dsDNA of the donor is nicked by endonuclease
- ssDNA binding protein binds to the DNA where one of the ends of a strand is bounds by different proteins among which SSB and RecA
- RecA induces the annealing betwen homologous DNA with complementary region on the recipient DNA(strand invasion)
- At the end, two DNA molecules are separated by by the resolvase, which cleaves and separates the DNA at the recombinant sites
Can different recombination sites be generated
Yes, depending on the break
Where can homologous recombination occur
- Between plasmids: Recombination between two plasmids leads to a chimeric form
- Between plasmid and recipient chromosome: when plasmid recombinates with a chromosome, integration occurs
Are homologous sequences short in the molecule that recombine
Yes
Where does site- specific recombination occurs
At specific target sites with specific DNA sequence
What are specific sites for site-specific recombination recognized by
Specific enzymes that are able to recognize and bind the site for site-specific recombination
What happens to mechanisms in site-specific recombination
-Viral genomes are inserted into the host chromosome
-specific genes are inserted during transposition
What is SSB in homologous recombination
Protein that keeps two parts of a nicked strand separated because they tend to connect
