Bacterial Genetics Lecture Sep 12 Flashcards
What are the two places a bacteria keeps its DNA?
GIve details about each.
Chromosome: they have 1 copy per cell, it’s circular dsDNA with 2000-4000 genes
Plasmid: it’s circular dsDNA outside the chromosome, usually about 1-20 copies in a cell with 5-100 genes on it
What is the difference between when the chromosome and plasmid replicate?
Chromosome replicates in concert with cell growth and division
The plasmid replicates independently of the chromosome
What genes does the chromosome carry?
What genes does the plasmid typically carry?
The chromosome carries the genes for all the functions of the cell.
THe plasmid carries virulence factors like toxin genes, ABx resistance genes, etc.
What are the three methods of genetic variation in bacteria?
- Mutation
- Gene Regulation
- Gene transfer
What are some causes of mutations?
- Things from th physical envrionment - UV light, x-rays, etc.
- The chemical nature of the nucleic acid bases (A looks like G, etc.)
- THe biological nature of the replication enzymes (proof-reading function reduces mistakes in nucleotide placement, but doesn’t catch everything)
- Mutagens (man-made molecules can increase the frequency of mutations)
What are the three main forms of gene transfer in bacteria?
What is a fourth that is connected to this?
Transformation
Conjugation
Transduction
and Transposition plays a role a well
What is transformation and how does it occur?
It’s a transfer of free-floating naked bacterial chromosomal DNA from a dead cell to a living cell
A bacterium that is expressing competence factor is able to take up any free floating DNA from the extracellular space into its cytosol
The recipient may incporportate that DNA into its DNA, exchanging its genes
What protein is required for a bacteria to undergo gene transfer thoruhg transformation?
competence factor
What sort of gene transfer is used in recombinant DNA work?
transformation
If you add a gene onto a plasmid through transformation and then add that to a bacterial susepction, the recipient bacterium will take the plasmid DNA bearing the gene in question and express the gene product. THe product can then be harvested and employed clinically.
What is conjugation?
Conjugation occurs between a male and female bacteria–it’s sexual trading of genetic information
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FOr conjugation purposes, what differentiates a male bacterium from a female bacterium?
A male bacterium will express F factor and have an F pili on its surface
A female will not express F factor or have a pilli
What are the three states the F factor can be found?
- Hfr bacterium: integrated into the host bacterial chromosome
- F+ bacterium: the f factor is free in the cytoplasm
- F’ bacterium : in rare conditions, a previoulsy integrated F factor may excise itself incorrectly from the host bacterial DNA and pick up a piece of host DNA
How does an Hfr bacterium transfer it’s F factor through HFr conjugation?
Remember that in Hfr bacteria the F factor is integrated into the host chromosome at one of a very few particular points
- Upon mating with the F- bacterium, the Hfr begins replciating the host bacterial chromosomal DNA, beginning just downstream of the integrated F factor
- THe newly replicated DNA is transferred to the F- bacterium
- Thus, the Hfr bacterium transfers the hos tbacterial chromosomal DNA first. THe F factor will only be transferred to the F- bacterium if conjugation continues for 90 minutes ofr more
- This would convert the F- bacterium to an Hfr bacterium
In conjugation, what triggers the replication of the F+ bacterial DNA?
F factor
How does an F+ bacterium that has an F factor that is free in the cytoplasm transfer it’s F factor through conjugation?
Conjugaton will occur as was seen for the Hfr bacterium, but ONLY THE F FACTOR IS EXCHANGED, convertin the F-bacterium to an F+ bacterium
So if a single F+ bacterium is introduced into a sulture of F- bacteria, essentially all of the bacteria will become F+
What is the main difference between Hfr conjugation and F+ conjugation?
The Hfr conjugation transfers some or all of the Hfr bacterium’s DNA–which may or may not include the F factor because that’s the last bit to be transferred
In F+ conjugation, only the F factor that is free floating will be transferred–it will be transferred every time conjugation occurs
Describe F’ conjugation.
Sometimes an integrated F factor will excise itself form the host bacterial chromosome to become an F+ bacterium
If the excision is incorrect, a bit of host bacterial DNA may be excised as part of the F factor, so it becomes the F’ factor
Conjugation then occurs as it would for F+ bacteria, only some of the host bacteria’s DNA is transferred in addition to the F factor
Why will F’ conjugation transfer only specific genes?
Because the F factors integrate at only a few specific sites on the host bacterial chromosome, the piece of DNA excised is limited to specific host genes that are located on either side of the integrated F factor
What allows F factors to integrate only at specific sites?
Transposons!
Transposons are sequences of DNA that can recognize complementary sequences in other DNA and “hop” from one complementary sequence ot another
F factor contains a transposon
Thus, F factor can use the transposon to integrate into the host DNA< but there are only a limited number of sites in the bacterial DNA that the F factor transposon will be able to recognize
What two genes within a transposon control the movement of the transposon?
Transposase and Resolvase
What do transposons freuqently contain?
ABx resistance
What is a resistance transfer factor (RFT)?
It is a plasmid that contain ABx resistance AND transposons
How are resistance transfer factors exchanged between bacteria?
Exchanged through conjugation similarly to F factors
How does transduction occur?
A bacteria is infected by a phage carried both viral and bacterial DNA that can be incorporated into the host genome.
What are the two types of bacteriophage?
virulent phage and temperate phage
Where is the genetic information for the diptheria toxin encoded?
in the episolin phage, which is a temperate phage
Describe a virulent phage.
Do they promote generalized or specific transduction?
These pahges infect a bacterium and replicate, killing the bacterium in the process
During their replication cycle, the phages may degrade bacterial chromosomal DNA to large or small pieces and one of these pieces may be put into one of the new phages being assembled
THen when the new phage with the bacterial DNA goes to infect another bacterium, it will transfer the bacterial DNA into the new bacterium in addition to its own viral DNA.
Because the DNA is degraded before it’s accidentally added to a phage, virulent phages give rise fo GENERALIZED TRANSDUCTION because any gene has an equal chance of being transferred.
WHat is a temperate phage?
What is another name for them?
Do they promote generalizes or specialized transduction?
Temperate phages infect a bacterium and can either replicate (kiling the bacterium in the lytic cycle) or integrate into the bacterial chromosome to be replicated along with the bacterial genome (lysogenic cycle)
Because of this, temperate phages are also called lysogenic phage.
WHen the phage is integrated, it’s called a prophage, which is in a lysogenic bacterium
THe integrated prophage is repressed by a phage protein and will stay repressed indefinitely, unless the paphage protein is degraded by UV light or heat
If the repressor protein is degraded, the prophage cuts itself out of the host chromosome by induction and being to replicate in a lytic cycle.
The new phage produced may end up getting a piece of bacterial DNA inside if the inducton step is sloppy
This gives rise fo SPECIALIZED TRANSDUCTION because only the genes that are on either side of the prophage have any chance of being transferred
WHat is the lysogenic cycle?
What is the lytic cycle?
THe lysogenic cycle is when the phage is integrated into the host DNA and replicates along with it. Eventually this will switch to the lytic phase when the repressor protein from the phage is degraded
The lytic cycle is when the phage actively replicates itself and then lyses the cell to get out.
What do pathogenicity islands contain?
They consist of transposons with ISs (a transposase enzyme gene) and groups of virulence associated genes
What proteins allow the specific transposons to integrate into specific areas on bacterial chromosomes?
Integrons
What are some comon features of PAI?
- they contain a high concentration of virulence genes
- they are in pathogenic gram negative bacteria and absentin the benign relatives
- they contain large distinct chromosomal regions with characteristic base composition different from the core genome
- They are frequently inserted adjacent to tRNA genes
- They have a high level of association with movbile genetic elements like integrase, transposase, ISs elements, ttransposons, plasmids, and phages
- They are genetically unstalbe if the mobility elements are present
- They are mosaic structure made of several acquisitions
What are 6 virulence factors that are often on PAIs?
- adherence factors
- toxins
- iron uptake systems
- ivasins
- secretion systems
Are PAIs typically found in gram negative or gram positive bacteria?
gram negative
What is bactofection?
When bacteria are used as a vehicle/vector to transport genetic information into a eukaryotic cell
