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Bacteriology lecture 04 Bacterial genetics mechanisms of gene transfer


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What are the three means by which genetic information can be transferred from one bacterium to another
Transformation (uptake of foreign DNA), transduction (Bacteriophage), conjugation (cell to cell contact)
If the transferred donor DNA is not itself a replicon, and therefore cannot replicate in the recipient bacterium, what must happen
Recombination between the donor DNA and the recipient genome is required to produce stable recombinant progeny
What does recombination in bacteria refer to
The replacement of one allele with another, rather than reassortment of two parental alleles as in a diploid organism
How is DNA taken up by a bacteria
1. Double stranded DNA molecules bind to specific receptors on recipient cells and are taken up by the recipient
2. One of the strands is degraded by an exonuclease
3. Single strand is integrated into the recipient chromosome by strand displacement and recombination
Which bacteria take up only DNA from their own species
Haemophilus influenzae and Neisseria gonorrhoeae. Uptake requires the presence of a specific sequence of about 10 nucleotides
What is transduction
Transfer of genetic material from one cell (the donor) to another (the recipient) via a bacterial virus as vector
What are the stages of the growth cycle of a virulent phage
1. Adsorption (highly specific interaction between phage attachment protein and phage receptor on bacteria)
2. Penetration of nucleic acid (separation of phage nucleic acid from capsid)
3. Viral reproduction (gene transcription, maturation, release)
For a temperate phage, what is the vegetative cycle
Similar to the lytic cycle
For a temperate phage, what is the prophage stage
Persistence of phage genome in host cell for many generations
What is a lysogen
A bacteria that is carrying prophage
What does integration of phage DNA in bacterial chromosome require
Lambda genes, such as integrase
In relation to bacteriophages, what is repression
Tighly controlled shutoff of extraneous genes of the phage vegetative (replicative) cycle.
For lambda phage, what is the only phage gene transcribed
The cI gene, which encodes the phage repressor, a cytoplasmic protein which binds to double stranded DNA of the prophage
Lysogens are immune to infection by the same phage. How?
Immunity is due to repressor acting on incoming phage DNA. Different phages make different repressors so immunity is specific
What is generalized transduction
Tranducing particles are produced during lytic infection of the donor cells by the transducing bacterial virus. In rare cases, random fragments of the donor host genome are packaged into the viral capsid in lieu of the virus genome
What is abortive transduction
The donor DNA is not integrated (recombined) into the recipient genome and is only passed linearly on to daughter cells
What are specialized transducing phages
They contain hybrid DNA molecules with genes from both the bacterial host and the temperate phage. They are produced by an anomalous excision of the prophage
What is lysogenic conversion
A consequence of lysogeny where phages can contribute new genetic characteristics to their host
What is conjugation
Transfer of genetic material from one cell (donor) to another (recipient) via a direct cell-to-cell contact
What is Hfr mating type (High frequency recombination)
The F factor can be stably integrated into the chromosome, resulting in the formation of an Hfr
What is F'
The F factor can be excised from the chromosome. Anomalous excision gives rise to an F' factor which is a hybrid DNA molecule carrying both F factor genes and bacterial genes
What is the fate of transferred DNA from Hfr donors
Donor DNA must recombine into recipient genome if it is to be stably inherited
What is homologous recombination
What is required for it to occur in E. coli
Reciprocal exchange between donor and recipient DNA that are homologous in nucleotide sequences. In E. coli, the recA gene is required
What is site-specific recombination
Breakage and rejoining at specific sites of the donor and recipient DNA molecules
What are plasmids
Extrachromosomal genetic elements. They have their own origins of replication, but require host enzymes for their replication and for expression (transcription and translation) of the genes located on the plasmids
What are resistance plasmids (R factors)
Plasmids that contain genes responsible for antibiotic resistance
What are the two functionally distinct parts of R factors
Resistance determinant (R determinant) and Resistance transfer factor (RTF)
What are transposable elements (transposons)
Elements that can move from one site to another site within a genome or between two genomes. R-determinants are an example
What is transposition
Process that does not require homology between the donor site and the recipient site and is independent of the RecA protein. Sequence specific enzymes (transposases) are required for this process
What are insertion sequences
They carry the gene for transposase, the enzyme that catalyzes transposition.
What are the two types of transposition
Cut and paste (direct or non-replicative); Replicative (formation of a co-integrate)
What is the consequence of the insertion of an insertion sequence
Insertion sequence (IS) may affect the expression of the gene into which it is inserted. It may also provide a new promoter for the gene downstream of the insertion site. They can therefore act as switches for turning a gene on or off
What are transposons (or Tn elements)
Larger than IS elements. In addition to a gene encoding transposase, they often contain genes unrelated to insertion function

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