Genetics I and II (1/4 and 1/7) Flashcards
What did the Griffiths experiment demonstrate?
That there was some exchange of information between the live, nonvirulent bacteria and the heat-killed, previously virulent bactera which caused the mouse to die
What did the Avery et al experiment show?
Avery et al showed that DNA is the thing that caused this transformation both on colonies & by doing Griffiths experiment with adding DNA instead of heat killed bacteria to nonvirulent strain of DNA → mouse dies
How is genetic material organized in bacteria?
Most bacteria contain a single chromosome + extrachromosomal elements
Some have megaplasmids/minichromosomes
Some contain >1 chromosomes
It’s less regulated/less complicated than human genetic material
What is a plasmid?
Extrachromosomal DNA, can be circular or linear, 2kb-100s’ of kb, nonessential & carries supplemental genetic info, can employ host functions for most DNA metabolism, virulence, antibiotic resistance, etc
How do point mutations introduce genetic variation in bacterium?
Point mutations that occur during replication/cell division can be harmful i.e. nonsense mutation or can be beneficial i.e. introduce a change to protein that makes it function better
Natural selection determines whether the new clone overtakes the original population
How do DNA rearrangements introduce genetic variation?
DNA rearrangements are shuffling of the genetic information to produce insertions, deletions, inversions, or other changes in structure. Can involve 100s or 5000 bp changes
Examples include: insertion of transposon that can turn off a repressor so a toxin gene isn’t regulated
adding an insertion sequence (IS) element that increase expression of beta lactamase so bacteria can grow even without presence of lactose
What are 3 mechanisms of horizontal transfer of genetic information in bacteria?
Transformation: release and uptake of naked DNA (leaves DNA sensitive to nucleases & recipient must have competence factor to bring the DNA into the cell)
Transduction: packaging and transfer of bacterial DNA by viruses
Conjugation: bacterial mating
What are the 2 pathways for transduction?
Lytic: virus replicates then bursts out of cells
Lysogenic: viral DNA gets incorporated into the host genome
What did we learn from the “U” tube experiment?
That a change in phenotype of bacteria is due to virus (DNAse was present so transduction was ruled out & cells didn’t touch so conjugation was ruled out)
What are the 2 types of transduction?
Generalized transduction: during a lytic pathway, an error during the lytic life cycle causes a DNA fragment from the host to be incorporated into the lytic bacteriophage. This bacteriophage goes on to infect another bacterium, thereby transferring DNA from one bacterium to another. Note that this bacteriophage (which mistakenly took the host DNA) cannot lyse the new cell, only transfer DNA to it
Specialized transduction: temperate bacteriophage (prophage- can’t lyse the cell) injects donor DNA, which sits in the chromosome, and doesn’t do anything for a while. When the virus assembles, it takes part of the host DNA which, when infects a new host, inserts into host chromosome
What is conjugation?
Transfer of DNA (plasmid) between living donor and recipient bacterium through a sex pilus
Conjugative plasmids have been isolated in 30 genera of bacteria
What is F+ conjugation?
The transfer of a large (95 kb) F+ plasmid which encodes only a sex pilus
Does not involve transfer of chromosomal DNA from donor to recipient
Involves a sex pilus
Other plasmids i.e. that which encodes antibiotic resistance can be transferred during this process
What is R-plasmid conjugation?
Conjugation of an R-plasmid, which contains multiple antibiotic resistance genes and encodes a sex pilus
Smaller genes i.e. 1 kb are transferred
What is conjugative transposition?
Transfer of a composite transposon, which carry genes i.e. for antibiotic resistance flanked on both sides by IS elements
This composite transposon excises from chromosome, forms a conjugative plasmid, which encodes machinery to be conjugated & can be transferred from cell to cell
10 kb
What did we learn about transduction from Corynebacterium diptheriae?
Avirulent strains of Corynebacterium diptheriae that are infected with a bacteriophage that contained a toxin encoding gene made the bacteria virulent
